Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

dev/bots/analyze_sample_code.dart

@@ -7,6 +7,7 @@
 // To run this, from the root of the Flutter repository:
 //   bin/cache/dart-sdk/bin/dart dev/bots/analyze_sample_code.dart
 
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:args/args.dart';
@@ -41,7 +42,7 @@ void main(List<String> arguments) {
   argParser.addOption(
     'interactive',
     abbr: 'i',
-    help: 'Analyzes the sample code in the specified file interactivly.',
+    help: 'Analyzes the sample code in the specified file interactively.',
   );
 
   final ArgResults parsedArguments = argParser.parse(arguments);
@@ -326,6 +327,8 @@ class SampleChecker {
   void _extractSamples(List<File> files, {Map<String, Section> sectionMap, Map<String, Sample> sampleMap, bool silent = false}) {
     final List<Section> sections = <Section>[];
     final List<Sample> samples = <Sample>[];
+    int dartpadCount = 0;
+    int sampleCount = 0;
 
     for (final File file in files) {
       final String relativeFilePath = path.relative(file.path, from: _flutterPackage.path);
@@ -430,6 +433,12 @@ class SampleChecker {
           } else if (sampleMatch != null) {
             inSnippet = sampleMatch != null && (sampleMatch[1] == 'sample' || sampleMatch[1] == 'dartpad');
             if (inSnippet) {
+              if (sampleMatch[1] == 'sample') {
+                sampleCount++;
+              }
+              if (sampleMatch[1] == 'dartpad') {
+                dartpadCount++;
+              }
               startLine = Line(
                 '',
                 filename: relativeFilePath,
@@ -455,7 +464,7 @@ class SampleChecker {
       }
     }
     if (!silent)
-      print('Found ${sections.length} sample code sections.');
+      print('Found ${sections.length} snippet code blocks, $sampleCount sample code sections, and $dartpadCount dartpad sections.');
     for (final Section section in sections) {
       final String path = _writeSection(section).path;
       if (sectionMap != null)
@@ -510,10 +519,10 @@ class SampleChecker {
   }
 
   /// Creates the configuration files necessary for the analyzer to consider
-  /// the temporary director a package, and sets which lint rules to enforce.
+  /// the temporary directory a package, and sets which lint rules to enforce.
   void _createConfigurationFiles(Directory directory) {
     final File pubSpec = File(path.join(directory.path, 'pubspec.yaml'))..createSync(recursive: true);
-    final File analysisOptions = File(path.join(directory.path, 'analysis_options.yaml'))..createSync(recursive: true);
+
     pubSpec.writeAsStringSync('''
 name: analyze_sample_code
 environment:
@@ -524,12 +533,9 @@ dependencies:
   flutter_test:
     sdk: flutter
 ''');
-    analysisOptions.writeAsStringSync('''
+
-linter:
+    // Copy in the analysis options from the Flutter root.
-  rules:
+    File(path.join(_flutterRoot,'analysis_options.yaml')).copySync(path.join(directory.path, 'analysis_options.yaml'));
-    - unnecessary_const
-    - unnecessary_new
-''');
   }
 
   /// Writes out a sample section to the disk and returns the file.
@@ -606,22 +612,30 @@ linter:
     final String kBullet = Platform.isWindows ? ' - ' : ' • ';
     // RegExp to match an error output line of the analyzer.
     final RegExp errorPattern = RegExp(
-      '^ +([a-z]+)$kBullet(.+)$kBullet(.+):([0-9]+):([0-9]+)$kBullet([-a-z_]+)\$',
+      '^ +(?<type>[a-z]+)'
+      '$kBullet(?<description>.+)'
+      '$kBullet(?<file>.+):(?<line>[0-9]+):(?<column>[0-9]+)'
+      '$kBullet(?<code>[-a-z_]+)\$',
       caseSensitive: false,
     );
     bool unknownAnalyzerErrors = false;
     final int headerLength = headers.length + 3;
     for (final String error in errors) {
-      final Match parts = errorPattern.matchAsPrefix(error);
+      final RegExpMatch match = errorPattern.firstMatch(error);
-      if (parts != null) {
+      if (match == null) {
-        final String message = parts[2];
+        stderr.writeln('Analyzer output: $error');
-        final File file = File(path.join(_tempDirectory.path, parts[3]));
+        unknownAnalyzerErrors = true;
+        continue;
+      }
+      final String type = match.namedGroup('type');
+      final String message = match.namedGroup('description');
+      final File file = File(path.join(_tempDirectory.path, match.namedGroup('file')));
       final List<String> fileContents = file.readAsLinesSync();
       final bool isSnippet = path.basename(file.path).startsWith('snippet.');
       final bool isSample = path.basename(file.path).startsWith('sample.');
-        final String line = parts[4];
+      final String line = match.namedGroup('line');
-        final String column = parts[5];
+      final String column = match.namedGroup('column');
-        final String errorCode = parts[6];
+      final String errorCode = match.namedGroup('code');
       final int lineNumber = int.parse(line, radix: 10) - (isSnippet ? headerLength : 0);
       final int columnNumber = int.parse(column, radix: 10);
 
@@ -630,6 +644,7 @@ linter:
         addAnalysisError(
           file,
           AnalysisError(
+            type,
             lineNumber,
             columnNumber,
             message,
@@ -652,6 +667,7 @@ linter:
         addAnalysisError(
           file,
           AnalysisError(
+            type,
             lineNumber,
             columnNumber,
             message,
@@ -665,6 +681,7 @@ linter:
           addAnalysisError(
             file,
             AnalysisError(
+              type,
               lineNumber,
               columnNumber,
               message,
@@ -692,6 +709,7 @@ linter:
               addAnalysisError(
                 file,
                 AnalysisError(
+                  type,
                   actualLine.line,
                   actualLine.indent + fileContents[lineNumber - 2].length - 1,
                   'Unexpected comma at end of sample code.',
@@ -704,6 +722,7 @@ linter:
             addAnalysisError(
               file,
               AnalysisError(
+                type,
                 lineNumber - 1,
                 columnNumber,
                 message,
@@ -716,6 +735,7 @@ linter:
           addAnalysisError(
             file,
             AnalysisError(
+              type,
               actualLine.line,
               actualLine.indent + columnNumber,
               message,
@@ -725,10 +745,6 @@ linter:
           );
         }
       }
-      } else {
-        stderr.writeln('Analyzer output: $error');
-        unknownAnalyzerErrors = true;
-      }
     }
     if (_exitCode == 1 && analysisErrors.isEmpty && !unknownAnalyzerErrors) {
       _exitCode = 0;
@@ -906,6 +922,7 @@ class Sample {
 /// Changes how it converts to a string based on the source of the error.
 class AnalysisError {
   const AnalysisError(
+    this.type,
     this.line,
     this.column,
     this.message,
@@ -914,6 +931,7 @@ class AnalysisError {
     this.sample,
   });
 
+  final String type;
   final int line;
   final int column;
   final String message;
@@ -924,19 +942,18 @@ class AnalysisError {
   @override
   String toString() {
     if (source != null) {
-      return '${source.toStringWithColumn(column)}\n>>> $message ($errorCode)';
+      return '${source.toStringWithColumn(column)}\n>>> $type: $message ($errorCode)';
     } else if (sample != null) {
       return 'In sample starting at '
           '${sample.start.filename}:${sample.start.line}:${sample.contents[line - 1]}\n'
-          '>>> $message ($errorCode)';
+          '>>> $type: $message ($errorCode)';
     } else {
-      return '<source unknown>:$line:$column\n>>> $message ($errorCode)';
+      return '<source unknown>:$line:$column\n>>> $type: $message ($errorCode)';
     }
   }
 }
 
 Future<void> _runInteractive(Directory tempDir, Directory flutterPackage, String filePath) async {
-  print('Starting up in interactive mode...');
   filePath = path.isAbsolute(filePath) ? filePath : path.join(path.current, filePath);
   final File file = File(filePath);
   if (!file.existsSync()) {
@@ -955,6 +972,7 @@ Future<void> _runInteractive(Directory tempDir, Directory flutterPackage, String
     });
     print('Using temp dir ${tempDir.path}');
   }
+  print('Starting up in interactive mode on ${path.relative(filePath, from: _flutterRoot)} ...');
 
   void analyze(SampleChecker checker, File file) {
     final Map<String, Section> sections = <String, Section>{};
@@ -976,8 +994,32 @@ Future<void> _runInteractive(Directory tempDir, Directory flutterPackage, String
     .._createConfigurationFiles(tempDir);
   analyze(checker, file);
 
-  Watcher(file.absolute.path).events.listen((_) {
+  print('Type "q" to quit, or "r" to delete temp dir and manually reload.');
+
+  void rerun() {
     print('\n\nRerunning...');
+    try {
       analyze(checker, file);
+    } on SampleCheckerException catch (e) {
+      print('Caught Exception (${e.runtimeType}), press "r" to retry:\n$e');
+    }
+  }
+
+  stdin.lineMode = false;
+  stdin.echoMode = false;
+  stdin.transform(utf8.decoder).listen((String input) {
+    switch (input) {
+      case 'q':
+        print('Exiting...');
+        exit(0);
+        break;
+      case 'r':
+        print('Deleting temp files...');
+        tempDir.deleteSync(recursive: true);
+        rerun();
+        break;
+    }
   });
+
+  Watcher(file.absolute.path).events.listen((_) => rerun());
 }

dev/bots/test/analyze_sample_code_test.dart

@@ -17,20 +17,32 @@ void main() {
       ..removeWhere((String line) => line.startsWith('Analyzer output:') || line.startsWith('Building flutter tool...'));
     expect(process.exitCode, isNot(equals(0)));
     expect(stderrLines, <String>[
+      'In sample starting at known_broken_documentation.dart:117:bool? _visible = true;',
+      '>>> info: Use late for private members with non-nullable type (use_late_for_private_fields_and_variables)',
       'In sample starting at known_broken_documentation.dart:117:      child: Text(title),',
-      '>>> The final variable \'title\' can\'t be read because it is potentially unassigned at this point (read_potentially_unassigned_final)',
+      '>>> error: The final variable \'title\' can\'t be read because it is potentially unassigned at this point (read_potentially_unassigned_final)',
       'known_broken_documentation.dart:30:9: new Opacity(',
-      '>>> Unnecessary new keyword (unnecessary_new)',
+      '>>> info: Unnecessary new keyword (unnecessary_new)',
       'known_broken_documentation.dart:62:9: new Opacity(',
-      '>>> Unnecessary new keyword (unnecessary_new)',
+      '>>> info: Unnecessary new keyword (unnecessary_new)',
+      'known_broken_documentation.dart:95:9: const text0 = Text(\'Poor wandering ones!\');',
+      '>>> info: Specify type annotations (always_specify_types)',
+      'known_broken_documentation.dart:103:9: const text1 = _Text(\'Poor wandering ones!\');',
+      '>>> info: Specify type annotations (always_specify_types)',
+      'known_broken_documentation.dart:111:9: final String? bar = \'Hello\';',
+      '>>> info: Prefer const over final for declarations (prefer_const_declarations)',
+      'known_broken_documentation.dart:111:23: final String? bar = \'Hello\';',
+      '>>> info: Use a non-nullable type for a final variable initialized with a non-nullable value (unnecessary_nullable_for_final_variable_declarations)',
+      'known_broken_documentation.dart:112:9: final int foo = null;',
+      '>>> info: Prefer const over final for declarations (prefer_const_declarations)',
       'known_broken_documentation.dart:112:25: final int foo = null;',
-      '>>> A value of type \'Null\' can\'t be assigned to a variable of type \'int\' (invalid_assignment)',
+      '>>> error: A value of type \'Null\' can\'t be assigned to a variable of type \'int\' (invalid_assignment)',
       '',
       'Found 2 sample code errors.',
       ''
     ]);
     expect(stdoutLines, <String>[
-      'Found 8 sample code sections.',
+      'Found 8 snippet code blocks, 0 sample code sections, and 2 dartpad sections.',
       'Starting analysis of code samples.',
       '',
     ]);

dev/bots/test/analyze_test.dart

@@ -28,7 +28,12 @@ Future<String> capture(AsyncVoidCallback callback, { int exitCode = 0 }) async {
   } finally {
     print = oldPrint;
   }
+  if (stdout.supportsAnsiEscapes) {
+    // Remove ANSI escapes when this test is running on a terminal.
+    return buffer.toString().replaceAll(RegExp(r'(\x9B|\x1B\[)[0-?]{1,3}[ -/]*[@-~]'), '');
+  } else {
     return buffer.toString();
+  }
 }
 
 void main() {

dev/snippets/config/templates/freeform.tmpl

@@ -2,6 +2,7 @@
 
 {{description}}
 
+{{code-dartImports}}
 {{code-imports}}
 
 {{code-main}}

dev/snippets/config/templates/stateful_widget.tmpl

@@ -2,20 +2,23 @@
 
 {{description}}
 
-import 'package:flutter/widgets.dart';
+{{code-dartImports}}
 
+import 'package:flutter/widgets.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
-    return WidgetsApp(
+    return const WidgetsApp(
       title: 'Flutter Code Sample',
       home: MyStatefulWidget(),
-      color: const Color(0xffffffff),
+      color: Color(0xffffffff),
     );
   }
 }
@@ -24,7 +27,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_cupertino.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/cupertino.dart';
+{{code-dartImports}}
 
+import 'package:flutter/cupertino.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
   Widget build(BuildContext context) {
-    return CupertinoApp(
+    return const CupertinoApp(
       title: _title,
       home: MyStatefulWidget(),
     );
@@ -25,7 +28,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_cupertino_page_scaffold.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/cupertino.dart';
+{{code-dartImports}}
 
+import 'package:flutter/cupertino.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
   Widget build(BuildContext context) {
-    return CupertinoApp(
+    return const CupertinoApp(
       title: _title,
       home: CupertinoPageScaffold(
         navigationBar: CupertinoNavigationBar(middle: const Text(_title)),
@@ -28,7 +31,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_cupertino_ticker.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/cupertino.dart';
+{{code-dartImports}}
 
+import 'package:flutter/cupertino.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
   Widget build(BuildContext context) {
-    return CupertinoApp(
+    return const CupertinoApp(
       title: _title,
       home: MyStatefulWidget(),
     );
@@ -25,7 +28,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_material.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
+    return const MaterialApp(
       title: _title,
       home: MyStatefulWidget(),
     );
@@ -25,7 +28,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_material_ticker.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
+    return const MaterialApp(
       title: _title,
       home: MyStatefulWidget(),
     );
@@ -25,7 +28,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_restoration.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     return WidgetsApp(
       title: 'Flutter Code Sample',
-      home: Center(
+      home: const Center(
         child: MyStatefulWidget(restorationId: 'main'),
       ),
       color: const Color(0xffffffff),
@@ -26,7 +29,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key, this.restorationId}) : super(key: key);
+  const MyStatefulWidget({Key? key, this.restorationId}) : super(key: key);
 
   final String? restorationId;
 

dev/snippets/config/templates/stateful_widget_scaffold.tmpl

@@ -2,14 +2,17 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
@@ -18,7 +21,7 @@ class MyApp extends StatelessWidget {
       title: _title,
       home: Scaffold(
         appBar: AppBar(title: const Text(_title)),
-        body: MyStatefulWidget(),
+        body: const MyStatefulWidget(),
       ),
     );
   }
@@ -28,7 +31,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_scaffold_center.tmpl

@@ -2,14 +2,17 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
@@ -18,7 +21,7 @@ class MyApp extends StatelessWidget {
       title: _title,
       home: Scaffold(
         appBar: AppBar(title: const Text(_title)),
-        body: Center(
+        body: const Center(
           child: MyStatefulWidget(),
         ),
       ),
@@ -30,7 +33,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_scaffold_center_freeform_state.tmpl

@@ -2,14 +2,17 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
@@ -18,7 +21,7 @@ class MyApp extends StatelessWidget {
       title: _title,
       home: Scaffold(
         appBar: AppBar(title: const Text(_title)),
-        body: Center(
+        body: const Center(
           child: MyStatefulWidget(),
         ),
       ),
@@ -30,7 +33,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_ticker.tmpl

@@ -2,20 +2,23 @@
 
 {{description}}
 
-import 'package:flutter/widgets.dart';
+{{code-dartImports}}
 
+import 'package:flutter/widgets.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
-    return WidgetsApp(
+    return const WidgetsApp(
       title: 'Flutter Code Sample',
       home: MyStatefulWidget(),
-      color: const Color(0xffffffff),
+      color: Color(0xffffffff),
     );
   }
 }
@@ -24,7 +27,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key? key}) : super(key: key);
+  const MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateless_widget.tmpl

@@ -2,21 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/widgets.dart';
+{{code-dartImports}}
 
+import 'package:flutter/widgets.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     return WidgetsApp(
       title: 'Flutter Code Sample',
-      builder: (BuildContext context, Widget navigator) {
+      builder: (BuildContext context, Widget? navigator) => const MyStatelessWidget(),
-        return MyStatelessWidget();
-      },
       color: const Color(0xffffffff),
     );
   }
@@ -26,7 +27,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key? key}) : super(key: key);
+  const MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_cupertino.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/cupertino.dart';
+{{code-dartImports}}
 
+import 'package:flutter/cupertino.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
   Widget build(BuildContext context) {
-    return CupertinoApp(
+    return const CupertinoApp(
       title: _title,
       home: MyStatelessWidget(),
     );
@@ -26,7 +29,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key? key}) : super(key: key);
+  const MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_cupertino_page_scaffold.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/cupertino.dart';
+{{code-dartImports}}
 
+import 'package:flutter/cupertino.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
   Widget build(BuildContext context) {
-    return CupertinoApp(
+    return const CupertinoApp(
       title: _title,
       home: CupertinoPageScaffold(
         navigationBar: CupertinoNavigationBar(middle: const Text(_title)),
@@ -29,7 +32,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key? key}) : super(key: key);
+  const MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_material.tmpl

@@ -2,19 +2,22 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
+    return const MaterialApp(
       title: _title,
       home: MyStatelessWidget(),
     );
@@ -26,7 +29,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key? key}) : super(key: key);
+  const MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_scaffold.tmpl

@@ -2,14 +2,17 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
@@ -18,7 +21,7 @@ class MyApp extends StatelessWidget {
       title: _title,
       home: Scaffold(
         appBar: AppBar(title: const Text(_title)),
-        body: MyStatelessWidget(),
+        body: const MyStatelessWidget(),
       ),
     );
   }
@@ -29,7 +32,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key? key}) : super(key: key);
+  const MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_scaffold_center.tmpl

@@ -2,14 +2,17 @@
 
 {{description}}
 
-import 'package:flutter/material.dart';
+{{code-dartImports}}
 
+import 'package:flutter/material.dart';
 {{code-imports}}
 
-void main() => runApp(new MyApp());
+void main() => runApp(const MyApp());
 
 /// This is the main application widget.
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   static const String _title = 'Flutter Code Sample';
 
   @override
@@ -18,7 +21,7 @@ class MyApp extends StatelessWidget {
       title: _title,
       home: Scaffold(
         appBar: AppBar(title: const Text(_title)),
-        body: Center(
+        body: const Center(
           child: MyStatelessWidget(),
         ),
       ),
@@ -31,7 +34,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key? key}) : super(key: key);
+  const MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

packages/flutter/lib/src/animation/animation.dart

@@ -138,7 +138,7 @@ abstract class Animation<T> extends Listenable implements ValueListenable<T> {
   /// final Animatable<Alignment> _tween = AlignmentTween(begin: Alignment.topLeft, end: Alignment.topRight)
   ///   .chain(CurveTween(curve: Curves.easeIn));
   /// // ...
-  /// Animation<Alignment> _alignment2 = _controller.drive(_tween);
+  /// final Animation<Alignment> _alignment2 = _controller.drive(_tween);
   /// ```
   /// {@end-tool}
   /// {@tool snippet}

packages/flutter/lib/src/animation/animation_controller.dart

@@ -136,7 +136,7 @@ enum AnimationBehavior {
 ///
 /// ```dart
 /// class Foo extends StatefulWidget {
-///   Foo({ Key? key, required this.duration }) : super(key: key);
+///   const Foo({ Key? key, required this.duration }) : super(key: key);
 ///
 ///   final Duration duration;
 ///

packages/flutter/lib/src/animation/curves.dart

@@ -339,8 +339,8 @@ class Cubic extends Curve {
 ///     Offset(0.93, 0.93),
 ///     Offset(0.05, 0.75),
 ///   ],
-///   startHandle: Offset(0.93, 0.93),
+///   startHandle: const Offset(0.93, 0.93),
-///   endHandle: Offset(0.18, 0.23),
+///   endHandle: const Offset(0.18, 0.23),
 ///   tension: 0.0,
 /// );
 ///
@@ -389,7 +389,7 @@ class Cubic extends Curve {
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     // Scale the path values to match the -1.0 to 1.0 domain of the Alignment widget.
-///     final Offset position = widget.path.transform(animation.value) * 2.0 - Offset(1.0, 1.0);
+///     final Offset position = widget.path.transform(animation.value) * 2.0 - const Offset(1.0, 1.0);
 ///     return Align(
 ///       alignment: Alignment(position.dx, position.dy),
 ///       child: widget.child,
@@ -411,7 +411,7 @@ class Cubic extends Curve {
 ///           backgroundColor: Colors.yellow,
 ///           child: DefaultTextStyle(
 ///             style: Theme.of(context).textTheme.headline6!,
-///             child: Text("B"), // Buzz, buzz!
+///             child: const Text('B'), // Buzz, buzz!
 ///           ),
 ///         ),
 ///       ),

packages/flutter/lib/src/animation/tween_sequence.dart

@@ -22,7 +22,7 @@ import 'tween.dart';
 /// for the next 20%, and then returns to 5.0 for the final 40%.
 ///
 /// ```dart
-/// final Animation<double> animation = TweenSequence(
+/// final Animation<double> animation = TweenSequence<double>(
 ///   <TweenSequenceItem<double>>[
 ///     TweenSequenceItem<double>(
 ///       tween: Tween<double>(begin: 5.0, end: 10.0)

packages/flutter/lib/src/cupertino/action_sheet.dart

@@ -105,6 +105,8 @@ const double _kDividerThickness = 1.0;
 ///
 /// ```dart
 /// class MyStatefulWidget extends StatefulWidget {
+///   const MyStatefulWidget({Key? key}) : super(key: key);
+///
 ///   @override
 ///   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
 /// }
@@ -116,12 +118,12 @@ const double _kDividerThickness = 1.0;
 ///       child: Center(
 ///         child: CupertinoButton(
 ///           onPressed: () {
-///             showCupertinoModalPopup(
+///             showCupertinoModalPopup<void>(
 ///               context: context,
 ///               builder: (BuildContext context) => CupertinoActionSheet(
 ///                 title: const Text('Title'),
 ///                 message: const Text('Message'),
-///                 actions: [
+///                 actions: <CupertinoActionSheetAction>[
 ///                   CupertinoActionSheetAction(
 ///                     child: const Text('Action One'),
 ///                     onPressed: () {
@@ -138,7 +140,7 @@ const double _kDividerThickness = 1.0;
 ///               ),
 ///             );
 ///           },
-///           child: Text('CupertinoActionSheet'),
+///           child: const Text('CupertinoActionSheet'),
 ///         ),
 ///       ),
 ///     );

packages/flutter/lib/src/cupertino/app.dart

@@ -57,10 +57,10 @@ import 'theme.dart';
 /// ![The CupertinoApp displays a CupertinoPageScaffold](https://flutter.github.io/assets-for-api-docs/assets/cupertino/basic_cupertino_app.png)
 ///
 /// ```dart
-/// CupertinoApp(
+/// const CupertinoApp(
 ///   home: CupertinoPageScaffold(
 ///     navigationBar: CupertinoNavigationBar(
-///       middle: const Text('Home'),
+///       middle: Text('Home'),
 ///     ),
 ///     child: Center(child: Icon(CupertinoIcons.share)),
 ///   ),
@@ -77,17 +77,17 @@ import 'theme.dart';
 /// CupertinoApp(
 ///   routes: <String, WidgetBuilder>{
 ///     '/': (BuildContext context) {
-///       return CupertinoPageScaffold(
+///       return const CupertinoPageScaffold(
 ///         navigationBar: CupertinoNavigationBar(
-///           middle: const Text('Home Route'),
+///           middle: Text('Home Route'),
 ///         ),
 ///         child: Center(child: Icon(CupertinoIcons.share)),
 ///       );
 ///     },
 ///     '/about': (BuildContext context) {
-///       return CupertinoPageScaffold(
+///       return const CupertinoPageScaffold(
 ///         navigationBar: CupertinoNavigationBar(
-///           middle: const Text('About Route'),
+///           middle: Text('About Route'),
 ///         ),
 ///         child: Center(child: Icon(CupertinoIcons.share)),
 ///       );
@@ -104,14 +104,14 @@ import 'theme.dart';
 /// ![The CupertinoApp displays a CupertinoPageScaffold with orange-colored icons](https://flutter.github.io/assets-for-api-docs/assets/cupertino/theme_cupertino_app.png)
 ///
 /// ```dart
-/// CupertinoApp(
+/// const CupertinoApp(
 ///   theme: CupertinoThemeData(
 ///     brightness: Brightness.dark,
 ///     primaryColor: CupertinoColors.systemOrange,
 ///   ),
 ///   home: CupertinoPageScaffold(
 ///     navigationBar: CupertinoNavigationBar(
-///       middle: const Text('CupertinoApp Theme'),
+///       middle: Text('CupertinoApp Theme'),
 ///     ),
 ///     child: Center(child: Icon(CupertinoIcons.share)),
 ///   ),
@@ -374,7 +374,7 @@ class CupertinoApp extends StatefulWidget {
   ///   return WidgetsApp(
   ///     actions: <Type, Action<Intent>>{
   ///       ... WidgetsApp.defaultActions,
-  ///       ActivateAction: CallbackAction(
+  ///       ActivateAction: CallbackAction<Intent>(
   ///         onInvoke: (Intent intent) {
   ///           // Do something here...
   ///           return null;

packages/flutter/lib/src/cupertino/colors.dart

@@ -601,7 +601,7 @@ class CupertinoColors {
 /// CupertinoButton(
 ///   child: child,
 ///   // CupertinoDynamicColor works out of box in a CupertinoButton.
-///   color: CupertinoDynamicColor.withBrightness(
+///   color: const CupertinoDynamicColor.withBrightness(
 ///     color: CupertinoColors.white,
 ///     darkColor: CupertinoColors.black,
 ///   ),

packages/flutter/lib/src/cupertino/context_menu.dart

@@ -86,7 +86,7 @@ enum _ContextMenuLocation {
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     body: Center(
-///       child: Container(
+///       child: SizedBox(
 ///         width: 100,
 ///         height: 100,
 ///         child: CupertinoContextMenu(

packages/flutter/lib/src/cupertino/form_row.dart

@@ -41,7 +41,7 @@ const EdgeInsetsGeometry _kDefaultPadding =
 ///
 /// ```dart
 /// class FlutterDemo extends StatefulWidget {
-///   FlutterDemo({Key? key}) : super(key: key);
+///   const FlutterDemo({Key? key}) : super(key: key);
 ///
 ///   @override
 ///   _FlutterDemoState createState() => _FlutterDemoState();
@@ -55,20 +55,20 @@ const EdgeInsetsGeometry _kDefaultPadding =
 ///     return CupertinoPageScaffold(
 ///       child: Center(
 ///         child: CupertinoFormSection(
-///           header: Text('SECTION 1'),
+///           header: const Text('SECTION 1'),
 ///           children: <Widget>[
 ///             CupertinoFormRow(
 ///               child: CupertinoSwitch(
-///                 value: this.toggleValue,
+///                 value: toggleValue,
-///                 onChanged: (value) {
+///                 onChanged: (bool value) {
 ///                   setState(() {
-///                     this.toggleValue = value;
+///                     toggleValue = value;
 ///                   });
 ///                 },
 ///               ),
-///               prefix: Text('Toggle'),
+///               prefix: const Text('Toggle'),
-///               helper: Text('Use your instincts'),
+///               helper: const Text('Use your instincts'),
-///               error: toggleValue ? Text('Cannot be true') : null,
+///               error: toggleValue ? const Text('Cannot be true') : null,
 ///             ),
 ///           ],
 ///         ),

packages/flutter/lib/src/cupertino/nav_bar.dart

@@ -189,6 +189,7 @@ bool _isTransitionable(BuildContext context) {
 ///
 ///
 /// ```dart
+/// @override
 /// Widget build(BuildContext context) {
 ///   return CupertinoPageScaffold(
 ///     navigationBar: CupertinoNavigationBar(
@@ -197,7 +198,7 @@ bool _isTransitionable(BuildContext context) {
 ///       middle: const Text('Sample Code'),
 ///     ),
 ///     child: Column(
-///       children: [
+///       children: <Widget>[
 ///         Container(height: 50, color: CupertinoColors.systemRed),
 ///         Container(height: 50, color: CupertinoColors.systemGreen),
 ///         Container(height: 50, color: CupertinoColors.systemBlue),

packages/flutter/lib/src/cupertino/page_scaffold.dart

@@ -26,15 +26,16 @@ import 'theme.dart';
 /// ```dart
 /// int _count = 0;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return CupertinoPageScaffold(
 ///     // Uncomment to change the background color
 ///     // backgroundColor: CupertinoColors.systemPink,
-///     navigationBar: CupertinoNavigationBar(
+///     navigationBar: const CupertinoNavigationBar(
 ///       middle: const Text('Sample Code'),
 ///     ),
 ///     child: ListView(
-///       children: [
+///       children: <Widget>[
 ///         CupertinoButton(
 ///           onPressed: () => setState(() => _count++),
 ///           child: const Icon(CupertinoIcons.add),

packages/flutter/lib/src/cupertino/refresh.dart

@@ -273,12 +273,12 @@ typedef RefreshCallback = Future<void> Function();
 /// adds a new item to the top of the list view.
 ///
 /// ```dart
-/// List<Color> colors = [
+/// List<Color> colors = <Color>[
 ///   CupertinoColors.systemYellow,
 ///   CupertinoColors.systemOrange,
 ///   CupertinoColors.systemPink
 /// ];
-/// List<Widget> items = [
+/// List<Widget> items = <Widget>[
 ///   Container(color: CupertinoColors.systemPink, height: 100.0),
 ///   Container(color: CupertinoColors.systemOrange, height: 100.0),
 ///   Container(color: CupertinoColors.systemYellow, height: 100.0),
@@ -296,7 +296,7 @@ typedef RefreshCallback = Future<void> Function();
 ///             refreshTriggerPullDistance: 100.0,
 ///             refreshIndicatorExtent: 60.0,
 ///             onRefresh: () async {
-///               await Future.delayed(Duration(milliseconds: 1000));
+///               await Future<void>.delayed(const Duration(milliseconds: 1000));
 ///               setState(() {
 ///                 items.insert(0, Container(color: colors[items.length % 3], height: 100.0));
 ///               });

packages/flutter/lib/src/cupertino/route.dart

@@ -1144,13 +1144,15 @@ class CupertinoModalPopupRoute<T> extends PopupRoute<T> {
 ///
 /// ```dart
 /// void main() {
-///   runApp(MyApp());
+///   runApp(const MyApp());
 /// }
 ///
 /// class MyApp extends StatelessWidget {
+///   const MyApp({Key? key}) : super(key: key);
+///
 ///   @override
 ///   Widget build(BuildContext context) {
-///     return CupertinoApp(
+///     return const CupertinoApp(
 ///       restorationScopeId: 'app',
 ///       home: MyHomePage(),
 ///     );
@@ -1158,13 +1160,15 @@ class CupertinoModalPopupRoute<T> extends PopupRoute<T> {
 /// }
 ///
 /// class MyHomePage extends StatelessWidget {
-///   static Route _modalBuilder(BuildContext context, Object? arguments) {
+///   const MyHomePage({Key? key}) : super(key: key);
-///     return CupertinoModalPopupRoute(
+///
+///   static Route<void> _modalBuilder(BuildContext context, Object? arguments) {
+///     return CupertinoModalPopupRoute<void>(
 ///       builder: (BuildContext context) {
 ///         return CupertinoActionSheet(
 ///           title: const Text('Title'),
 ///           message: const Text('Message'),
-///           actions: [
+///           actions: <CupertinoActionSheetAction>[
 ///             CupertinoActionSheetAction(
 ///               child: const Text('Action One'),
 ///               onPressed: () {
@@ -1306,13 +1310,15 @@ Widget _buildCupertinoDialogTransitions(BuildContext context, Animation<double>
 ///
 /// ```dart
 /// void main() {
-///   runApp(MyApp());
+///   runApp(const MyApp());
 /// }
 ///
 /// class MyApp extends StatelessWidget {
+///   const MyApp({Key? key}) : super(key: key);
+///
 ///   @override
 ///   Widget build(BuildContext context) {
-///     return CupertinoApp(
+///     return const CupertinoApp(
 ///       restorationScopeId: 'app',
 ///       home: MyHomePage(),
 ///     );
@@ -1320,6 +1326,8 @@ Widget _buildCupertinoDialogTransitions(BuildContext context, Animation<double>
 /// }
 ///
 /// class MyHomePage extends StatelessWidget {
+///   const MyHomePage({Key? key}) : super(key: key);
+///
 ///   static Route<Object?> _dialogBuilder(BuildContext context, Object? arguments) {
 ///     return CupertinoDialogRoute<void>(
 ///       context: context,

packages/flutter/lib/src/cupertino/search_field.dart

@@ -25,6 +25,8 @@ import 'text_field.dart';
 ///
 /// ```dart
 /// class MyPrefilledSearch extends StatefulWidget {
+///   const MyPrefilledSearch({Key? key}) : super(key: key);
+///
 ///   @override
 ///   _MyPrefilledSearchState createState() => _MyPrefilledSearchState();
 /// }
@@ -54,6 +56,8 @@ import 'text_field.dart';
 ///
 /// ```dart
 /// class MyPrefilledSearch extends StatefulWidget {
+///   const MyPrefilledSearch({Key? key}) : super(key: key);
+///
 ///   @override
 ///   _MyPrefilledSearchState createState() => _MyPrefilledSearchState();
 /// }
@@ -62,11 +66,11 @@ import 'text_field.dart';
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return CupertinoSearchTextField(
-///       onChanged: (value) {
+///       onChanged: (String value) {
-///         print("The text has changed to: " + value);
+///         print('The text has changed to: $value');
 ///       },
-///       onSubmitted: (value) {
+///       onSubmitted: (String value) {
-///         print("Submitted text: " + value);
+///         print('Submitted text: $value');
 ///       },
 ///     );
 ///   }

packages/flutter/lib/src/cupertino/segmented_control.dart

@@ -127,12 +127,14 @@ class CupertinoSegmentedControl<T extends Object> extends StatefulWidget {
   ///
   /// ```dart
   /// class SegmentedControlExample extends StatefulWidget {
+  ///   const SegmentedControlExample({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///   State createState() => SegmentedControlExampleState();
   /// }
   ///
   /// class SegmentedControlExampleState extends State<SegmentedControlExample> {
-  ///   final Map<int, Widget> children = const {
+  ///   final Map<int, Widget> children = const <int, Widget>{
   ///     0: Text('Child 1'),
   ///     1: Text('Child 2'),
   ///   };
@@ -141,8 +143,7 @@ class CupertinoSegmentedControl<T extends Object> extends StatefulWidget {
   ///
   ///   @override
   ///   Widget build(BuildContext context) {
-  ///     return Container(
+  ///     return CupertinoSegmentedControl<int>(
-  ///       child: CupertinoSegmentedControl<int>(
   ///       children: children,
   ///       onValueChanged: (int newValue) {
   ///         setState(() {
@@ -150,7 +151,6 @@ class CupertinoSegmentedControl<T extends Object> extends StatefulWidget {
   ///         });
   ///       },
   ///       groupValue: currentValue,
-  ///       ),
   ///     );
   ///   }
   /// }

packages/flutter/lib/src/cupertino/sliding_segmented_control.dart

@@ -352,12 +352,14 @@ class CupertinoSlidingSegmentedControl<T> extends StatefulWidget {
   ///
   /// ```dart
   /// class SegmentedControlExample extends StatefulWidget {
+  ///   const SegmentedControlExample({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///   State createState() => SegmentedControlExampleState();
   /// }
   ///
   /// class SegmentedControlExampleState extends State<SegmentedControlExample> {
-  ///   final Map<int, Widget> children = const {
+  ///   final Map<int, Widget> children = const <int, Widget>{
   ///     0: Text('Child 1'),
   ///     1: Text('Child 2'),
   ///   };
@@ -366,8 +368,7 @@ class CupertinoSlidingSegmentedControl<T> extends StatefulWidget {
   ///
   ///   @override
   ///   Widget build(BuildContext context) {
-  ///     return Container(
+  ///     return CupertinoSlidingSegmentedControl<int>(
-  ///       child: CupertinoSlidingSegmentedControl<int>(
   ///       children: children,
   ///       onValueChanged: (int? newValue) {
   ///         setState(() {
@@ -375,7 +376,6 @@ class CupertinoSlidingSegmentedControl<T> extends StatefulWidget {
   ///         });
   ///       },
   ///       groupValue: currentValue,
-  ///       ),
   ///     );
   ///   }
   /// }

packages/flutter/lib/src/cupertino/switch.dart

@@ -35,7 +35,7 @@ import 'thumb_painter.dart';
 /// ```dart
 /// MergeSemantics(
 ///   child: ListTile(
-///     title: Text('Lights'),
+///     title: const Text('Lights'),
 ///     trailing: CupertinoSwitch(
 ///       value: _lights,
 ///       onChanged: (bool value) { setState(() { _lights = value; }); },

packages/flutter/lib/src/cupertino/tab_scaffold.dart

@@ -20,6 +20,8 @@ import 'theme.dart';
 ///
 /// ```dart
 /// class MyCupertinoTabScaffoldPage extends StatefulWidget {
+///   const MyCupertinoTabScaffoldPage({Key? key}) : super(key: key);
+///
 ///   @override
 ///   _CupertinoTabScaffoldPageState createState() => _CupertinoTabScaffoldPageState();
 /// }
@@ -31,7 +33,7 @@ import 'theme.dart';
 ///   Widget build(BuildContext context) {
 ///     return CupertinoTabScaffold(
 ///       tabBar: CupertinoTabBar(
-///         items: <BottomNavigationBarItem> [
+///         items: const <BottomNavigationBarItem> [
 ///           // ...
 ///         ],
 ///       ),
@@ -143,7 +145,7 @@ class CupertinoTabController extends ChangeNotifier {
 /// ```dart
 /// CupertinoTabScaffold(
 ///   tabBar: CupertinoTabBar(
-///     items: <BottomNavigationBarItem> [
+///     items: const <BottomNavigationBarItem> [
 ///       // ...
 ///     ],
 ///   ),

packages/flutter/lib/src/cupertino/text_field.dart

@@ -149,6 +149,8 @@ class _CupertinoTextFieldSelectionGestureDetectorBuilder extends TextSelectionGe
 ///
 /// ```dart
 /// class MyPrefilledText extends StatefulWidget {
+///   const MyPrefilledText({Key? key}) : super(key: key);
+///
 ///   @override
 ///   _MyPrefilledTextState createState() => _MyPrefilledTextState();
 /// }

packages/flutter/lib/src/cupertino/text_form_field_row.dart

@@ -58,7 +58,7 @@ import 'text_field.dart';
 ///
 /// ```dart
 /// CupertinoTextFormFieldRow(
-///   prefix: Text('Username'),
+///   prefix: const Text('Username'),
 ///   onSaved: (String? value) {
 ///     // This optional block of code can be used to run
 ///     // code when the user saves the form.
@@ -79,6 +79,7 @@ import 'text_field.dart';
 /// ```
 ///
 /// ```dart
+/// @override
 /// Widget build(BuildContext context) {
 ///   return CupertinoPageScaffold(
 ///     child: Center(
@@ -88,12 +89,12 @@ import 'text_field.dart';
 ///           Form.of(primaryFocus!.context!)?.save();
 ///         },
 ///         child: CupertinoFormSection.insetGrouped(
-///           header: Text('SECTION 1'),
+///           header: const Text('SECTION 1'),
 ///           children: List<Widget>.generate(5, (int index) {
 ///             return CupertinoTextFormFieldRow(
-///               prefix: Text('Enter text'),
+///               prefix: const Text('Enter text'),
 ///               placeholder: 'Enter text',
-///               validator: (value) {
+///               validator: (String? value) {
 ///                 if (value == null || value.isEmpty) {
 ///                   return 'Please enter a value';
 ///                 }

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -1763,8 +1763,8 @@ abstract class DiagnosticsNode {
 /// of an actual property of the object:
 ///
 /// ```dart
-/// var table = MessageProperty('table size', '$columns\u00D7$rows');
+/// MessageProperty table = MessageProperty('table size', '$columns\u00D7$rows');
-/// var usefulness = MessageProperty('usefulness ratio', 'no metrics collected yet (never painted)');
+/// MessageProperty usefulness = MessageProperty('usefulness ratio', 'no metrics collected yet (never painted)');
 /// ```
 /// {@end-tool}
 /// {@tool snippet}
@@ -1773,7 +1773,7 @@ abstract class DiagnosticsNode {
 /// concrete value that is a string:
 ///
 /// ```dart
-/// var name = StringProperty('name', _name);
+/// StringProperty name = StringProperty('name', _name);
 /// ```
 /// {@end-tool}
 ///
@@ -3068,7 +3068,7 @@ class DiagnosticPropertiesBuilder {
 }
 
 // Examples can assume:
-// class ExampleSuperclass with Diagnosticable { late String message; late double stepWidth; late double scale; late double paintExtent; late double hitTestExtent; late double paintExtend; late double maxWidth; late bool primary; late double progress; late int maxLines; late Duration duration; late int depth; late dynamic boxShadow; late dynamic style; late bool hasSize; late Matrix4 transform; Map<Listenable, VoidCallback>? handles; late Color color; late bool obscureText; late ImageRepeat repeat; late Size size; late Widget widget; late bool isCurrent; late bool keepAlive; late TextAlign textAlign; }
+// class ExampleSuperclass with Diagnosticable { late String message; late double stepWidth; late double scale; late double paintExtent; late double hitTestExtent; late double paintExtend; late double maxWidth; late bool primary; late double progress; late int maxLines; late Duration duration; late int depth; Iterable<BoxShadow>? boxShadow; late DiagnosticsTreeStyle style; late bool hasSize; late Matrix4 transform; Map<Listenable, VoidCallback>? handles; late Color color; late bool obscureText; late ImageRepeat repeat; late Size size; late Widget widget; late bool isCurrent; late bool keepAlive; late TextAlign textAlign; }
 
 /// A mixin class for providing string and [DiagnosticsNode] debug
 /// representations describing the properties of an object.

packages/flutter/lib/src/foundation/licenses.dart

@@ -71,7 +71,7 @@ enum _LicenseEntryWithLineBreaksParserState {
 /// ```dart
 /// void initMyLibrary() {
 ///   LicenseRegistry.addLicense(() async* {
-///     yield LicenseEntryWithLineBreaks(<String>['my_library'], '''
+///     yield const LicenseEntryWithLineBreaks(<String>['my_library'], '''
 /// Copyright 2016 The Sample Authors. All rights reserved.
 ///
 /// Redistribution and use in source and binary forms, with or without

packages/flutter/lib/src/material/about.dart

@@ -52,7 +52,7 @@ import 'theme.dart';
 ///  Widget build(BuildContext context) {
 ///    final TextStyle textStyle = Theme.of(context).textTheme.bodyText2!;
 ///    final List<Widget> aboutBoxChildren = <Widget>[
-///      SizedBox(height: 24),
+///      const SizedBox(height: 24),
 ///      RichText(
 ///        text: TextSpan(
 ///          children: <TextSpan>[
@@ -77,14 +77,14 @@ import 'theme.dart';
 ///
 ///    return Scaffold(
 ///      appBar: AppBar(
-///        title: Text('Show About Example'),
+///        title: const Text('Show About Example'),
 ///      ),
 ///      drawer: Drawer(
 ///        child: SingleChildScrollView(
 ///          child: SafeArea(
 ///            child: AboutListTile(
-///              icon: Icon(Icons.info),
+///              icon: const Icon(Icons.info),
-///              applicationIcon: FlutterLogo(),
+///              applicationIcon: const FlutterLogo(),
 ///              applicationName: 'Show About Example',
 ///              applicationVersion: 'August 2019',
 ///              applicationLegalese: '\u{a9} 2014 The Flutter Authors',
@@ -95,11 +95,11 @@ import 'theme.dart';
 ///      ),
 ///      body: Center(
 ///        child: ElevatedButton(
-///          child: Text('Show About Example'),
+///          child: const Text('Show About Example'),
 ///          onPressed: () {
 ///            showAboutDialog(
 ///              context: context,
-///              applicationIcon: FlutterLogo(),
+///              applicationIcon: const FlutterLogo(),
 ///              applicationName: 'Show About Example',
 ///              applicationVersion: 'August 2019',
 ///              applicationLegalese: '\u{a9} 2014 The Flutter Authors',

packages/flutter/lib/src/material/app.dart

@@ -613,7 +613,7 @@ class MaterialApp extends StatefulWidget {
   ///   return WidgetsApp(
   ///     actions: <Type, Action<Intent>>{
   ///       ... WidgetsApp.defaultActions,
-  ///       ActivateAction: CallbackAction(
+  ///       ActivateAction: CallbackAction<Intent>(
   ///         onInvoke: (Intent intent) {
   ///           // Do something here...
   ///           return null;

packages/flutter/lib/src/material/app_bar.dart

@@ -115,7 +115,7 @@ class _ToolbarContainerLayout extends SingleChildLayoutDelegate {
 ///           icon: const Icon(Icons.navigate_next),
 ///           tooltip: 'Go to the next page',
 ///           onPressed: () {
-///             Navigator.push(context, MaterialPageRoute(
+///             Navigator.push(context, MaterialPageRoute<void>(
 ///               builder: (BuildContext context) {
 ///                 return Scaffold(
 ///                   appBar: AppBar(
@@ -317,10 +317,10 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   ///     primary: true,
   ///     slivers: <Widget>[
   ///       SliverAppBar(
-  ///         title: Text('Hello World'),
+  ///         title: const Text('Hello World'),
   ///         actions: <Widget>[
   ///           IconButton(
-  ///             icon: Icon(Icons.shopping_cart),
+  ///             icon: const Icon(Icons.shopping_cart),
   ///             tooltip: 'Open shopping cart',
   ///             onPressed: () {
   ///               // handle the press
@@ -1273,7 +1273,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
 /// ```
 ///
 /// ```dart
-/// void main() => runApp(MyApp());
+/// void main() => runApp(const MyApp());
 ///
 /// class MyApp extends StatefulWidget {
 ///   const MyApp({Key? key}) : super(key: key);
@@ -1296,20 +1296,20 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
 ///         body: CustomScrollView(
 ///           slivers: <Widget>[
 ///             SliverAppBar(
-///               pinned: this._pinned,
+///               pinned: _pinned,
-///               snap: this._snap,
+///               snap: _snap,
-///               floating: this._floating,
+///               floating: _floating,
 ///               expandedHeight: 160.0,
-///               flexibleSpace: FlexibleSpaceBar(
+///               flexibleSpace: const FlexibleSpaceBar(
-///                 title: const Text("SliverAppBar"),
+///                 title: Text('SliverAppBar'),
 ///                 background: FlutterLogo(),
 ///               ),
 ///             ),
-///             SliverToBoxAdapter(
+///             const SliverToBoxAdapter(
 ///               child: Center(
-///                 child: Container(
+///                 child: SizedBox(
 ///                   height: 2000,
-///                   child: const Text("Scroll to see SliverAppBar in effect ."),
+///                   child: const Text('Scroll to see SliverAppBar in effect .'),
 ///                 ),
 ///               ),
 ///             ),
@@ -1325,10 +1325,10 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
 ///                   Switch(
 ///                     onChanged: (bool val) {
 ///                       setState(() {
-///                         this._pinned = val;
+///                         _pinned = val;
 ///                       });
 ///                     },
-///                     value: this._pinned,
+///                     value: _pinned,
 ///                   ),
 ///                 ],
 ///               ),
@@ -1338,12 +1338,12 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
 ///                   Switch(
 ///                     onChanged: (bool val) {
 ///                       setState(() {
-///                         this._snap = val;
+///                         _snap = val;
 ///                         //Snapping only applies when the app bar is floating.
-///                         this._floating = this._floating || val;
+///                         _floating = _floating || val;
 ///                       });
 ///                     },
-///                     value: this._snap,
+///                     value: _snap,
 ///                   ),
 ///                 ],
 ///               ),
@@ -1353,15 +1353,15 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
 ///                   Switch(
 ///                     onChanged: (bool val) {
 ///                       setState(() {
-///                         this._floating = val;
+///                         _floating = val;
-///                         if (this._snap == true) {
+///                         if (_snap == true) {
-///                           if (this._floating != true) {
+///                           if (_floating != true) {
-///                             this._snap = false;
+///                             _snap = false;
 ///                           }
 ///                         }
 ///                       });
 ///                     },
-///                     value: this._floating,
+///                     value: _floating,
 ///                   ),
 ///                 ],
 ///               ),

packages/flutter/lib/src/material/autocomplete.dart

@@ -57,6 +57,7 @@ import 'text_form_field.dart';
 /// ```
 ///
 /// ```dart
+/// @immutable
 /// class User {
 ///   const User({
 ///     required this.email,
@@ -73,8 +74,9 @@ import 'text_form_field.dart';
 ///
 ///   @override
 ///   bool operator ==(Object other) {
-///     if (other.runtimeType != runtimeType)
+///     if (other.runtimeType != runtimeType) {
 ///       return false;
+///     }
 ///     return other is User
 ///         && other.name == name
 ///         && other.email == email;
@@ -85,9 +87,9 @@ import 'text_form_field.dart';
 /// }
 ///
 /// class AutocompleteBasicUserExample extends StatelessWidget {
-///   AutocompleteBasicUserExample({Key? key}) : super(key: key);
+///   const AutocompleteBasicUserExample({Key? key}) : super(key: key);
 ///
-///   static final List<User> _userOptions = <User>[
+///   static const List<User> _userOptions = <User>[
 ///     User(name: 'Alice', email: 'alice@example.com'),
 ///     User(name: 'Bob', email: 'bob@example.com'),
 ///     User(name: 'Charlie', email: 'charlie123@gmail.com'),

packages/flutter/lib/src/material/bottom_app_bar.dart

@@ -27,7 +27,7 @@ import 'theme.dart';
 ///     color: Colors.white,
 ///     child: bottomAppBarContents,
 ///   ),
-///   floatingActionButton: FloatingActionButton(onPressed: null),
+///   floatingActionButton: const FloatingActionButton(onPressed: null),
 /// )
 /// ```
 /// {@end-tool}
@@ -42,20 +42,20 @@ import 'theme.dart';
 ///
 /// ```dart
 /// void main() {
-///   runApp(BottomAppBarDemo());
+///   runApp(const BottomAppBarDemo());
 /// }
 ///
 /// class BottomAppBarDemo extends StatefulWidget {
-///   const BottomAppBarDemo();
+///   const BottomAppBarDemo({Key? key}) : super(key: key);
 ///
 ///   @override
 ///   State createState() => _BottomAppBarDemoState();
 /// }
 ///
 /// class _BottomAppBarDemoState extends State<BottomAppBarDemo> {
-///   var _showFab = true;
+///   bool _showFab = true;
-///   var _showNotch = true;
+///   bool _showNotch = true;
-///   var _fabLocation = FloatingActionButtonLocation.endDocked;
+///   FloatingActionButtonLocation _fabLocation = FloatingActionButtonLocation.endDocked;
 ///
 ///   void _onShowNotchChanged(bool value) {
 ///     setState(() {
@@ -69,9 +69,9 @@ import 'theme.dart';
 ///     });
 ///   }
 ///
-///   void _onFabLocationChanged(dynamic value) {
+///   void _onFabLocationChanged(FloatingActionButtonLocation? value) {
 ///     setState(() {
-///       _fabLocation = value;
+///       _fabLocation = value ?? FloatingActionButtonLocation.endDocked;
 ///     });
 ///   }
 ///
@@ -81,55 +81,47 @@ import 'theme.dart';
 ///       home: Scaffold(
 ///         appBar: AppBar(
 ///           automaticallyImplyLeading: false,
-///           title: Text("Bottom App Bar Demo"),
+///           title: const Text('Bottom App Bar Demo'),
 ///         ),
 ///         body: ListView(
 ///           padding: const EdgeInsets.only(bottom: 88),
-///           children: [
+///           children: <Widget>[
 ///             SwitchListTile(
-///               title: Text(
+///               title: const Text(
-///                 "Floating Action Button",
+///                 'Floating Action Button',
 ///               ),
 ///               value: _showFab,
 ///               onChanged: _onShowFabChanged,
 ///             ),
 ///             SwitchListTile(
-///               title: Text("Notch"),
+///               title: const Text('Notch'),
 ///               value: _showNotch,
 ///               onChanged: _onShowNotchChanged,
 ///             ),
-///             Padding(
+///             const Padding(
-///               padding: const EdgeInsets.all(16),
+///               padding: EdgeInsets.all(16),
-///               child: Text("Floating action button position"),
+///               child: Text('Floating action button position'),
 ///             ),
 ///             RadioListTile<FloatingActionButtonLocation>(
-///               title: Text(
+///               title: const Text('Docked - End'),
-///                 "Docked - End",
-///               ),
 ///               value: FloatingActionButtonLocation.endDocked,
 ///               groupValue: _fabLocation,
 ///               onChanged: _onFabLocationChanged,
 ///             ),
 ///             RadioListTile<FloatingActionButtonLocation>(
-///               title: Text(
+///               title: const Text('Docked - Center'),
-///                 "Docked - Center",
-///               ),
 ///               value: FloatingActionButtonLocation.centerDocked,
 ///               groupValue: _fabLocation,
 ///               onChanged: _onFabLocationChanged,
 ///             ),
 ///             RadioListTile<FloatingActionButtonLocation>(
-///               title: Text(
+///               title: const Text('Floating - End'),
-///                 "Floating - End",
-///               ),
 ///               value: FloatingActionButtonLocation.endFloat,
 ///               groupValue: _fabLocation,
 ///               onChanged: _onFabLocationChanged,
 ///             ),
 ///             RadioListTile<FloatingActionButtonLocation>(
-///               title: Text(
+///               title: const Text('Floating - Center'),
-///                 "Floating - Center",
-///               ),
 ///               value: FloatingActionButtonLocation.centerFloat,
 ///               groupValue: _fabLocation,
 ///               onChanged: _onFabLocationChanged,
@@ -140,7 +132,7 @@ import 'theme.dart';
 ///             ? FloatingActionButton(
 ///                 onPressed: () {},
 ///                 child: const Icon(Icons.add),
-///                 tooltip: "Create",
+///                 tooltip: 'Create',
 ///               )
 ///             : null,
 ///         floatingActionButtonLocation: _fabLocation,
@@ -160,9 +152,9 @@ import 'theme.dart';
 ///   });
 ///
 ///   final FloatingActionButtonLocation fabLocation;
-///   final dynamic shape;
+///   final NotchedShape? shape;
 ///
-///   static final centerLocations = <FloatingActionButtonLocation>[
+///   static final List<FloatingActionButtonLocation> centerLocations = <FloatingActionButtonLocation>[
 ///     FloatingActionButtonLocation.centerDocked,
 ///     FloatingActionButtonLocation.centerFloat,
 ///   ];
@@ -175,7 +167,7 @@ import 'theme.dart';
 ///       child: IconTheme(
 ///         data: IconThemeData(color: Theme.of(context).colorScheme.onPrimary),
 ///         child: Row(
-///           children: [
+///           children: <Widget>[
 ///             IconButton(
 ///               tooltip: 'Open navigation menu',
 ///               icon: const Icon(Icons.menu),
@@ -183,12 +175,12 @@ import 'theme.dart';
 ///             ),
 ///             if (centerLocations.contains(fabLocation)) const Spacer(),
 ///             IconButton(
-///               tooltip: "Search",
+///               tooltip: 'Search',
 ///               icon: const Icon(Icons.search),
 ///               onPressed: () {},
 ///             ),
 ///             IconButton(
-///               tooltip: "Favorite",
+///               tooltip: 'Favorite',
 ///               icon: const Icon(Icons.favorite),
 ///               onPressed: () {},
 ///             ),

packages/flutter/lib/src/material/card.dart

@@ -78,7 +78,7 @@ import 'theme.dart';
 ///         onTap: () {
 ///           print('Card tapped.');
 ///         },
-///         child: Container(
+///         child: const SizedBox(
 ///           width: 300,
 ///           height: 100,
 ///           child: Text('A card that can be tapped'),

packages/flutter/lib/src/material/checkbox.dart

@@ -39,6 +39,8 @@ import 'toggleable.dart';
 ///
 /// ```dart
 /// bool isChecked = false;
+///
+/// @override
 /// Widget build(BuildContext context) {
 ///   Color getColor(Set<MaterialState> states) {
 ///     const Set<MaterialState> interactiveStates = <MaterialState>{
@@ -55,9 +57,9 @@ import 'toggleable.dart';
 ///     checkColor: Colors.white,
 ///     fillColor: MaterialStateProperty.resolveWith(getColor),
 ///     value: isChecked,
-///     onChanged: (val) {
+///     onChanged: (bool? value) {
 ///       setState(() {
-///         isChecked = !isChecked;
+///         isChecked = value!;
 ///       });
 ///     },
 ///   );

packages/flutter/lib/src/material/checkbox_list_tile.dart

@@ -99,11 +99,12 @@ import 'theme_data.dart';
 /// ```dart preamble
 /// class LinkedLabelCheckbox extends StatelessWidget {
 ///   const LinkedLabelCheckbox({
+///     Key? key,
 ///     required this.label,
 ///     required this.padding,
 ///     required this.value,
 ///     required this.onChanged,
-///   });
+///   }) : super(key: key);
 ///
 ///   final String label;
 ///   final EdgeInsets padding;
@@ -120,7 +121,7 @@ import 'theme_data.dart';
 ///             child: RichText(
 ///               text: TextSpan(
 ///                 text: label,
-///                 style: TextStyle(
+///                 style: const TextStyle(
 ///                   color: Colors.blueAccent,
 ///                   decoration: TextDecoration.underline,
 ///                 ),
@@ -179,11 +180,12 @@ import 'theme_data.dart';
 /// ```dart preamble
 /// class LabeledCheckbox extends StatelessWidget {
 ///   const LabeledCheckbox({
+///     Key? key,
 ///     required this.label,
 ///     required this.padding,
 ///     required this.value,
 ///     required this.onChanged,
-///   });
+///   }) : super(key: key);
 ///
 ///   final String label;
 ///   final EdgeInsets padding;

packages/flutter/lib/src/material/chip.dart

@@ -232,6 +232,8 @@ abstract class DeletableChipAttributes {
   /// }
   ///
   /// class CastList extends StatefulWidget {
+  ///   const CastList({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///   State createState() => CastListState();
   /// }
@@ -275,7 +277,7 @@ abstract class DeletableChipAttributes {
   /// ```dart
   /// @override
   /// Widget build(BuildContext context) {
-  ///   return CastList();
+  ///   return const CastList();
   /// }
   /// ```
   /// {@end-tool}
@@ -383,6 +385,8 @@ abstract class SelectableChipAttributes {
   ///
   /// ```dart
   /// class Wood extends StatefulWidget {
+  ///   const Wood({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///   State<StatefulWidget> createState() => WoodState();
   /// }
@@ -519,6 +523,8 @@ abstract class TappableChipAttributes {
   ///
   /// ```dart
   /// class Blacksmith extends StatelessWidget {
+  ///   const Blacksmith({Key? key}) : super(key: key);
+  ///
   ///   void startHammering() {
   ///     print('bang bang bang');
   ///   }
@@ -567,9 +573,9 @@ abstract class TappableChipAttributes {
 /// Chip(
 ///   avatar: CircleAvatar(
 ///     backgroundColor: Colors.grey.shade800,
-///     child: Text('AB'),
+///     child: const Text('AB'),
 ///   ),
-///   label: Text('Aaron Burr'),
+///   label: const Text('Aaron Burr'),
 /// )
 /// ```
 /// {@end-tool}
@@ -716,9 +722,9 @@ class Chip extends StatelessWidget implements ChipAttributes, DeletableChipAttri
 /// InputChip(
 ///   avatar: CircleAvatar(
 ///     backgroundColor: Colors.grey.shade800,
-///     child: Text('AB'),
+///     child: const Text('AB'),
 ///   ),
-///   label: Text('Aaron Burr'),
+///   label: const Text('Aaron Burr'),
 ///   onPressed: () {
 ///     print('I am the one thing in life.');
 ///   }
@@ -916,6 +922,8 @@ class InputChip extends StatelessWidget
 ///
 /// ```dart
 /// class MyThreeOptions extends StatefulWidget {
+///   const MyThreeOptions({Key? key}) : super(key: key);
+///
 ///   @override
 ///   _MyThreeOptionsState createState() => _MyThreeOptionsState();
 /// }
@@ -1106,6 +1114,8 @@ class ChoiceChip extends StatelessWidget
 /// }
 ///
 /// class CastFilter extends StatefulWidget {
+///   const CastFilter({Key? key}) : super(key: key);
+///
 ///   @override
 ///   State createState() => CastFilterState();
 /// }
@@ -1117,7 +1127,7 @@ class ChoiceChip extends StatelessWidget
 ///     const ActorFilterEntry('Eliza Hamilton', 'EH'),
 ///     const ActorFilterEntry('James Madison', 'JM'),
 ///   ];
-///   List<String> _filters = <String>[];
+///   final List<String> _filters = <String>[];
 ///
 ///   Iterable<Widget> get actorWidgets sync* {
 ///     for (final ActorFilterEntry actor in _cast) {
@@ -1331,11 +1341,11 @@ class FilterChip extends StatelessWidget
 /// ActionChip(
 ///   avatar: CircleAvatar(
 ///     backgroundColor: Colors.grey.shade800,
-///     child: Text('AB'),
+///     child: const Text('AB'),
 ///   ),
-///   label: Text('Aaron Burr'),
+///   label: const Text('Aaron Burr'),
 ///   onPressed: () {
-///     print("If you stand for nothing, Burr, what’ll you fall for?");
+///     print('If you stand for nothing, Burr, what’ll you fall for?');
 ///   }
 /// )
 /// ```

packages/flutter/lib/src/material/chip_theme.dart

@@ -65,6 +65,8 @@ class ChipTheme extends InheritedTheme {
   ///
   /// ```dart
   /// class Spaceship extends StatelessWidget {
+  ///   const Spaceship({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///   Widget build(BuildContext context) {
   ///     return ChipTheme(
@@ -122,6 +124,8 @@ class ChipTheme extends InheritedTheme {
 ///
 /// ```dart
 /// class CarColor extends StatefulWidget {
+///   const CarColor({Key? key}) : super(key: key);
+///
 ///   @override
 ///   State createState() => _CarColorState();
 /// }
@@ -134,7 +138,7 @@ class ChipTheme extends InheritedTheme {
 ///     return ChipTheme(
 ///       data: ChipTheme.of(context).copyWith(backgroundColor: Colors.lightBlue),
 ///       child: ChoiceChip(
-///         label: Text('Light Blue'),
+///         label: const Text('Light Blue'),
 ///         onSelected: (bool value) {
 ///           setState(() {
 ///             _color = value ? Colors.lightBlue : Colors.red;

packages/flutter/lib/src/material/circle_avatar.dart

@@ -46,7 +46,7 @@ import 'theme.dart';
 /// ```dart
 /// CircleAvatar(
 ///   backgroundColor: Colors.brown.shade800,
-///   child: Text('AH'),
+///   child: const Text('AH'),
 /// )
 /// ```
 /// {@end-tool}

packages/flutter/lib/src/material/data_table.dart

@@ -373,7 +373,7 @@ class DataCell {
 ///
 /// ```dart
 /// static const int numItems = 10;
-/// List<bool> selected = List<bool>.generate(numItems, (index) => false);
+/// List<bool> selected = List<bool>.generate(numItems, (int index) => false);
 ///
 /// @override
 /// Widget build(BuildContext context) {
@@ -387,17 +387,18 @@ class DataCell {
 ///       ],
 ///       rows: List<DataRow>.generate(
 ///         numItems,
-///         (index) => DataRow(
+///         (int index) => DataRow(
 ///           color: MaterialStateProperty.resolveWith<Color?>((Set<MaterialState> states) {
 ///             // All rows will have the same selected color.
 ///             if (states.contains(MaterialState.selected))
 ///               return Theme.of(context).colorScheme.primary.withOpacity(0.08);
 ///             // Even rows will have a grey color.
-///             if (index % 2 == 0)
+///             if (index.isEven) {
 ///               return Colors.grey.withOpacity(0.3);
+///             }
 ///             return null;  // Use default value for other states and odd rows.
 ///           }),
-///           cells: [DataCell(Text('Row $index'))],
+///           cells: <DataCell>[ DataCell(Text('Row $index')) ],
 ///           selected: selected[index],
 ///           onSelectChanged: (bool? value) {
 ///             setState(() {

packages/flutter/lib/src/material/dialog.dart

@@ -194,10 +194,10 @@ class Dialog extends StatelessWidget {
 ///     barrierDismissible: false, // user must tap button!
 ///     builder: (BuildContext context) {
 ///       return AlertDialog(
-///         title: Text('AlertDialog Title'),
+///         title: const Text('AlertDialog Title'),
 ///         content: SingleChildScrollView(
 ///           child: ListBody(
-///             children: <Widget>[
+///             children: const <Widget>[
 ///               Text('This is a demo alert dialog.'),
 ///               Text('Would you like to approve of this message?'),
 ///             ],
@@ -205,7 +205,7 @@ class Dialog extends StatelessWidget {
 ///         ),
 ///         actions: <Widget>[
 ///           TextButton(
-///             child: Text('Approve'),
+///             child: const Text('Approve'),
 ///             onPressed: () {
 ///               Navigator.of(context).pop();
 ///             },
@@ -232,7 +232,7 @@ class Dialog extends StatelessWidget {
 ///       builder: (BuildContext context) => AlertDialog(
 ///         title: const Text('AlertDialog Tilte'),
 ///         content: const Text('AlertDialog description'),
-///         actions: [
+///         actions: <Widget>[
 ///           TextButton(
 ///             onPressed: () => Navigator.pop(context, 'Cancel'),
 ///             child: const Text('Cancel'),
@@ -244,7 +244,7 @@ class Dialog extends StatelessWidget {
 ///         ],
 ///       ),
 ///     ),
-///     child: Text('Show Dialog'),
+///     child: const Text('Show Dialog'),
 ///   );
 /// }
 ///
@@ -367,13 +367,13 @@ class AlertDialog extends StatelessWidget {
   /// This is an example of a set of actions aligned with the content widget.
   /// ```dart
   /// AlertDialog(
-  ///   title: Text('Title'),
+  ///   title: const Text('Title'),
   ///   content: Container(width: 200, height: 200, color: Colors.green),
   ///   actions: <Widget>[
-  ///     ElevatedButton(onPressed: () {}, child: Text('Button 1')),
+  ///     ElevatedButton(onPressed: () {}, child: const Text('Button 1')),
-  ///     ElevatedButton(onPressed: () {}, child: Text('Button 2')),
+  ///     ElevatedButton(onPressed: () {}, child: const Text('Button 2')),
   ///   ],
-  ///   actionsPadding: EdgeInsets.symmetric(horizontal: 8.0),
+  ///   actionsPadding: const EdgeInsets.symmetric(horizontal: 8.0),
   /// )
   /// ```
   /// {@end-tool}
@@ -1009,13 +1009,15 @@ Widget _buildMaterialDialogTransitions(BuildContext context, Animation<double> a
 ///
 /// ```dart
 /// void main() {
-///   runApp(MyApp());
+///   runApp(const MyApp());
 /// }
 ///
 /// class MyApp extends StatelessWidget {
+///   const MyApp({Key? key}) : super(key: key);
+///
 ///   @override
 ///   Widget build(BuildContext context) {
-///     return MaterialApp(
+///     return const MaterialApp(
 ///       restorationScopeId: 'app',
 ///       title: 'Restorable Routes Demo',
 ///       home: MyHomePage(),
@@ -1024,6 +1026,8 @@ Widget _buildMaterialDialogTransitions(BuildContext context, Animation<double> a
 /// }
 ///
 /// class MyHomePage extends StatelessWidget {
+///   const MyHomePage({Key? key}) : super(key: key);
+///
 ///   static Route<Object?> _dialogBuilder(BuildContext context, Object? arguments) {
 ///     return DialogRoute<void>(
 ///       context: context,

packages/flutter/lib/src/material/divider.dart

@@ -140,7 +140,7 @@ class Divider extends StatelessWidget {
   /// {@tool snippet}
   ///
   /// ```dart
-  /// Divider(
+  /// const Divider(
   ///   color: Colors.deepOrange,
   /// )
   /// ```
@@ -332,7 +332,7 @@ class VerticalDivider extends StatelessWidget {
   /// {@tool snippet}
   ///
   /// ```dart
-  /// Divider(
+  /// const Divider(
   ///   color: Colors.deepOrange,
   /// )
   /// ```

packages/flutter/lib/src/material/dropdown.dart

@@ -760,10 +760,10 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 /// Widget build(BuildContext context) {
 ///   return DropdownButton<String>(
 ///     value: dropdownValue,
-///     icon: Icon(Icons.arrow_downward),
+///     icon: const Icon(Icons.arrow_downward),
 ///     iconSize: 24,
 ///     elevation: 16,
-///     style: TextStyle(
+///     style: const TextStyle(
 ///       color: Colors.deepPurple
 ///     ),
 ///     underline: Container(
@@ -1002,12 +1002,12 @@ class DropdownButton<T> extends StatefulWidget {
   ///           dropdownValue = newValue!;
   ///         });
   ///       },
-  ///       style: TextStyle(color: Colors.blue),
+  ///       style: const TextStyle(color: Colors.blue),
   ///       selectedItemBuilder: (BuildContext context) {
   ///         return options.map((String value) {
   ///           return Text(
   ///             dropdownValue,
-  ///             style: TextStyle(color: Colors.white),
+  ///             style: const TextStyle(color: Colors.white),
   ///           );
   ///         }).toList();
   ///       },

packages/flutter/lib/src/material/elevated_button.dart

@@ -52,9 +52,10 @@ import 'theme_data.dart';
 /// This sample produces an enabled and a disabled ElevatedButton.
 ///
 /// ```dart
+/// @override
 /// Widget build(BuildContext context) {
 ///   final ButtonStyle style =
-///     ElevatedButton.styleFrom(textStyle: TextStyle(fontSize: 20));
+///     ElevatedButton.styleFrom(textStyle: const TextStyle(fontSize: 20));
 ///
 ///   return Center(
 ///     child: Column(

packages/flutter/lib/src/material/expansion_panel.dart

@@ -166,7 +166,7 @@ class ExpansionPanelRadio extends ExpansionPanel {
 /// }
 ///
 /// List<Item> generateItems(int numberOfItems) {
-///   return List.generate(numberOfItems, (int index) {
+///   return List<Item>.generate(numberOfItems, (int index) {
 ///     return Item(
 ///       headerValue: 'Panel $index',
 ///       expandedValue: 'This is item number $index',
@@ -176,7 +176,7 @@ class ExpansionPanelRadio extends ExpansionPanel {
 /// ```
 ///
 /// ```dart
-/// List<Item> _data = generateItems(8);
+/// final List<Item> _data = generateItems(8);
 ///
 /// @override
 /// Widget build(BuildContext context) {
@@ -203,11 +203,11 @@ class ExpansionPanelRadio extends ExpansionPanel {
 ///         },
 ///         body: ListTile(
 ///           title: Text(item.expandedValue),
-///           subtitle: Text('To delete this panel, tap the trash can icon'),
+///           subtitle: const Text('To delete this panel, tap the trash can icon'),
-///           trailing: Icon(Icons.delete),
+///           trailing: const Icon(Icons.delete),
 ///           onTap: () {
 ///             setState(() {
-///               _data.removeWhere((currentItem) => item == currentItem);
+///               _data.removeWhere((Item currentItem) => item == currentItem);
 ///             });
 ///           }
 ///         ),
@@ -270,7 +270,7 @@ class ExpansionPanelList extends StatefulWidget {
   /// }
   ///
   /// List<Item> generateItems(int numberOfItems) {
-  ///   return List.generate(numberOfItems, (int index) {
+  ///   return List<Item>.generate(numberOfItems, (int index) {
   ///     return Item(
   ///       id: index,
   ///       headerValue: 'Panel $index',
@@ -281,7 +281,7 @@ class ExpansionPanelList extends StatefulWidget {
   /// ```
   ///
   /// ```dart
-  /// List<Item> _data = generateItems(8);
+  /// final List<Item> _data = generateItems(8);
   ///
   /// @override
   /// Widget build(BuildContext context) {
@@ -305,11 +305,11 @@ class ExpansionPanelList extends StatefulWidget {
   ///         },
   ///         body: ListTile(
   ///           title: Text(item.expandedValue),
-  ///           subtitle: Text('To delete this panel, tap the trash can icon'),
+  ///           subtitle: const Text('To delete this panel, tap the trash can icon'),
-  ///           trailing: Icon(Icons.delete),
+  ///           trailing: const Icon(Icons.delete),
   ///           onTap: () {
   ///             setState(() {
-  ///               _data.removeWhere((currentItem) => item == currentItem);
+  ///               _data.removeWhere((Item currentItem) => item == currentItem);
   ///             });
   ///           }
   ///         )

packages/flutter/lib/src/material/feedback.dart

@@ -32,6 +32,8 @@ import 'theme.dart';
 ///
 /// ```dart
 /// class WidgetWithWrappedHandler extends StatelessWidget {
+///   const WidgetWithWrappedHandler({Key? key}) : super(key: key);
+///
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return GestureDetector(
@@ -58,6 +60,8 @@ import 'theme.dart';
 ///
 /// ```dart
 /// class WidgetWithExplicitCall extends StatelessWidget {
+///   const WidgetWithExplicitCall({Key? key}) : super(key: key);
+///
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return GestureDetector(

packages/flutter/lib/src/material/flexible_space_bar.dart

@@ -66,9 +66,11 @@ enum StretchMode {
 /// import 'package:flutter/material.dart';
 /// ```
 /// ```dart
-/// void main() => runApp(MaterialApp(home: MyApp()));
+/// void main() => runApp(const MaterialApp(home: MyApp()));
 ///
 /// class MyApp extends StatelessWidget {
+///   const MyApp({Key? key}) : super(key: key);
+///
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return Scaffold(
@@ -79,11 +81,11 @@ enum StretchMode {
 ///             stretch: true,
 ///             onStretchTrigger: () {
 ///               // Function callback for stretch
-///               return Future.value();
+///               return Future<void>.value();
 ///             },
 ///             expandedHeight: 300.0,
 ///             flexibleSpace: FlexibleSpaceBar(
-///               stretchModes: <StretchMode>[
+///               stretchModes: const <StretchMode>[
 ///                 StretchMode.zoomBackground,
 ///                 StretchMode.blurBackground,
 ///                 StretchMode.fadeTitle,
@@ -92,7 +94,7 @@ enum StretchMode {
 ///               title: const Text('Flight Report'),
 ///               background: Stack(
 ///                 fit: StackFit.expand,
-///                 children: [
+///                 children: <Widget>[
 ///                   Image.network(
 ///                     'https://flutter.github.io/assets-for-api-docs/assets/widgets/owl-2.jpg',
 ///                     fit: BoxFit.cover,
@@ -114,7 +116,8 @@ enum StretchMode {
 ///             ),
 ///           ),
 ///           SliverList(
-///             delegate: SliverChildListDelegate([
+///             delegate: SliverChildListDelegate(
+///               const <Widget>[
 ///                 ListTile(
 ///                   leading: Icon(Icons.wb_sunny),
 ///                   title: Text('Sunday'),
@@ -126,7 +129,8 @@ enum StretchMode {
 ///                   subtitle: Text('sunny, h: 80, l: 65'),
 ///                 ),
 ///                 // ListTiles++
-///             ]),
+///               ],
+///             ),
 ///           ),
 ///         ],
 ///       ),

packages/flutter/lib/src/material/floating_action_button.dart

@@ -69,14 +69,14 @@ class _DefaultHeroTag {
 ///     appBar: AppBar(
 ///       title: const Text('Floating Action Button'),
 ///     ),
-///     body: Center(
+///     body: const Center(
-///       child: const Text('Press the button below!')
+///       child: Text('Press the button below!')
 ///     ),
 ///     floatingActionButton: FloatingActionButton(
 ///       onPressed: () {
 ///         // Add your onPressed code here!
 ///       },
-///       child: Icon(Icons.navigation),
+///       child: const Icon(Icons.navigation),
 ///       backgroundColor: Colors.green,
 ///     ),
 ///   );
@@ -97,15 +97,15 @@ class _DefaultHeroTag {
 ///     appBar: AppBar(
 ///       title: const Text('Floating Action Button Label'),
 ///     ),
-///     body: Center(
+///     body: const Center(
-///       child: const Text('Press the button with a label below!'),
+///       child: Text('Press the button with a label below!'),
 ///     ),
 ///     floatingActionButton: FloatingActionButton.extended(
 ///       onPressed: () {
 ///         // Add your onPressed code here!
 ///       },
-///       label: Text('Approve'),
+///       label: const Text('Approve'),
-///       icon: Icon(Icons.thumb_up),
+///       icon: const Icon(Icons.thumb_up),
 ///       backgroundColor: Colors.pink,
 ///     ),
 ///   );

packages/flutter/lib/src/material/floating_action_button_location.dart

@@ -461,12 +461,12 @@ abstract class FloatingActionButtonLocation {
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     appBar: AppBar(
-///       title: Text('Home page'),
+///       title: const Text('Home page'),
 ///     ),
 ///     floatingActionButton: FloatingActionButton(
 ///       onPressed: () { print('FAB pressed.'); },
 ///       tooltip: 'Increment',
-///       child: Icon(Icons.add),
+///       child: const Icon(Icons.add),
 ///     ),
 ///     floatingActionButtonLocation: AlmostEndFloatFabLocation(),
 ///   );

packages/flutter/lib/src/material/icon_button.dart

@@ -52,12 +52,13 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 /// ```
 ///
 /// ```dart
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Column(
 ///     mainAxisSize: MainAxisSize.min,
 ///     children: <Widget>[
 ///       IconButton(
-///         icon: Icon(Icons.volume_up),
+///         icon: const Icon(Icons.volume_up),
 ///         tooltip: 'Increase volume by 10',
 ///         onPressed: () {
 ///           setState(() {
@@ -94,6 +95,7 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 /// ![](https://flutter.github.io/assets-for-api-docs/assets/material/icon_button_background.png)
 ///
 /// ```dart
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Material(
 ///     color: Colors.white,
@@ -104,7 +106,7 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 ///           shape: CircleBorder(),
 ///         ),
 ///         child: IconButton(
-///           icon: Icon(Icons.android),
+///           icon: const Icon(Icons.android),
 ///           color: Colors.white,
 ///           onPressed: () {},
 ///         ),

packages/flutter/lib/src/material/ink_decoration.dart

@@ -59,7 +59,7 @@ import 'material.dart';
 ///       height: 100.0,
 ///       child: InkWell(
 ///         onTap: () { /* ... */ },
-///         child: Center(
+///         child: const Center(
 ///           child: Text('YELLOW'),
 ///         )
 ///       ),
@@ -78,17 +78,23 @@ import 'material.dart';
 ///   color: Colors.grey[800],
 ///   child: Center(
 ///     child: Ink.image(
-///       image: AssetImage('cat.jpeg'),
+///       image: const AssetImage('cat.jpeg'),
 ///       fit: BoxFit.cover,
 ///       width: 300.0,
 ///       height: 200.0,
 ///       child: InkWell(
 ///         onTap: () { /* ... */ },
-///         child: Align(
+///         child: const Align(
 ///           alignment: Alignment.topLeft,
 ///           child: Padding(
-///             padding: const EdgeInsets.all(10.0),
+///             padding: EdgeInsets.all(10.0),
-///             child: Text('KITTEN', style: TextStyle(fontWeight: FontWeight.w900, color: Colors.white)),
+///             child: Text(
+///               'KITTEN',
+///               style: TextStyle(
+///                 fontWeight: FontWeight.w900,
+///                 color: Colors.white,
+///               ),
+///             ),
 ///           ),
 ///         )
 ///       ),

packages/flutter/lib/src/material/ink_well.dart

@@ -1182,11 +1182,12 @@ class _InkResponseState extends State<_InkResponseStateWidget>
 /// ```dart
 /// double sideLength = 50;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return AnimatedContainer(
 ///     height: sideLength,
 ///     width: sideLength,
-///     duration: Duration(seconds: 2),
+///     duration: const Duration(seconds: 2),
 ///     curve: Curves.easeIn,
 ///     child: Material(
 ///       color: Colors.yellow,

packages/flutter/lib/src/material/input_decorator.dart

@@ -2415,7 +2415,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
 ///
 /// ```dart
 /// Widget build(BuildContext context) {
-///   return TextField(
+///   return const TextField(
 ///     decoration: InputDecoration(
 ///       icon: Icon(Icons.send),
 ///       hintText: 'Hint Text',
@@ -2438,7 +2438,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
 ///
 /// ```dart
 /// Widget build(BuildContext context) {
-///   return TextField(
+///   return const TextField(
 ///     decoration: InputDecoration.collapsed(
 ///       hintText: 'Hint Text',
 ///       border: OutlineInputBorder(),
@@ -2458,7 +2458,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
 ///
 /// ```dart
 /// Widget build(BuildContext context) {
-///   return TextField(
+///   return const TextField(
 ///     decoration: InputDecoration(
 ///       hintText: 'Hint Text',
 ///       errorText: 'Error Text',
@@ -2875,7 +2875,7 @@ class InputDecoration {
   ///     padding: const EdgeInsets.symmetric(horizontal: 8.0),
   ///     child: Column(
   ///       mainAxisAlignment: MainAxisAlignment.center,
-  ///       children: <Widget>[
+  ///       children: const <Widget>[
   ///         TextField(
   ///           decoration: InputDecoration(
   ///             hintText: 'Normal Icon Constraints',
@@ -3043,7 +3043,7 @@ class InputDecoration {
   ///     padding: const EdgeInsets.symmetric(horizontal: 8.0),
   ///     child: Column(
   ///       mainAxisAlignment: MainAxisAlignment.center,
-  ///       children: <Widget>[
+  ///       children: const <Widget>[
   ///         TextField(
   ///           decoration: InputDecoration(
   ///             hintText: 'Normal Icon Constraints',

packages/flutter/lib/src/material/list_tile.dart

@@ -399,12 +399,12 @@ enum ListTileControlAffinity {
 ///     child: Container(
 ///       width: 48,
 ///       height: 48,
-///       padding: EdgeInsets.symmetric(vertical: 4.0),
+///       padding: const EdgeInsets.symmetric(vertical: 4.0),
 ///       alignment: Alignment.center,
-///       child: CircleAvatar(),
+///       child: const CircleAvatar(),
 ///     ),
 ///   ),
-///   title: Text('title'),
+///   title: const Text('title'),
 ///   dense: false,
 /// ),
 /// ```
@@ -426,11 +426,12 @@ enum ListTileControlAffinity {
 /// ```dart preamble
 /// class CustomListItem extends StatelessWidget {
 ///   const CustomListItem({
+///     Key? key,
 ///     required this.thumbnail,
 ///     required this.title,
 ///     required this.user,
 ///     required this.viewCount,
-///   });
+///   }) : super(key: key);
 ///
 ///   final Widget thumbnail;
 ///   final String title;
@@ -547,7 +548,7 @@ enum ListTileControlAffinity {
 ///
 /// ```dart preamble
 /// class _ArticleDescription extends StatelessWidget {
-///   _ArticleDescription({
+///   const _ArticleDescription({
 ///     Key? key,
 ///     required this.title,
 ///     required this.subtitle,
@@ -573,7 +574,7 @@ enum ListTileControlAffinity {
 ///             crossAxisAlignment: CrossAxisAlignment.start,
 ///             children: <Widget>[
 ///               Text(
-///                 '$title',
+///                 title,
 ///                 maxLines: 2,
 ///                 overflow: TextOverflow.ellipsis,
 ///                 style: const TextStyle(
@@ -582,7 +583,7 @@ enum ListTileControlAffinity {
 ///               ),
 ///               const Padding(padding: EdgeInsets.only(bottom: 2.0)),
 ///               Text(
-///                 '$subtitle',
+///                 subtitle,
 ///                 maxLines: 2,
 ///                 overflow: TextOverflow.ellipsis,
 ///                 style: const TextStyle(
@@ -600,7 +601,7 @@ enum ListTileControlAffinity {
 ///             mainAxisAlignment: MainAxisAlignment.end,
 ///             children: <Widget>[
 ///               Text(
-///                 '$author',
+///                 author,
 ///                 style: const TextStyle(
 ///                   fontSize: 12.0,
 ///                   color: Colors.black87,
@@ -622,7 +623,7 @@ enum ListTileControlAffinity {
 /// }
 ///
 /// class CustomListItemTwo extends StatelessWidget {
-///   CustomListItemTwo({
+///   const CustomListItemTwo({
 ///     Key? key,
 ///     required this.thumbnail,
 ///     required this.title,
@@ -683,7 +684,7 @@ enum ListTileControlAffinity {
 ///         ),
 ///         title: 'Flutter 1.0 Launch',
 ///         subtitle:
-///           'Flutter continues to improve and expand its horizons.'
+///           'Flutter continues to improve and expand its horizons. '
 ///           'This text should max out at two lines and clip',
 ///         author: 'Dash',
 ///         publishDate: 'Dec 28',

packages/flutter/lib/src/material/material_state.dart

@@ -109,11 +109,11 @@ typedef MaterialPropertyResolver<T> = T Function(Set<MaterialState> states);
 ///
 /// ```dart
 /// class MyColor extends MaterialStateColor {
+///   const MyColor() : super(_defaultColor);
+///
 ///   static const int _defaultColor = 0xcafefeed;
 ///   static const int _pressedColor = 0xdeadbeef;
 ///
-///   const MyColor() : super(_defaultColor);
-///
 ///   @override
 ///   Color resolve(Set<MaterialState> states) {
 ///     if (states.contains(MaterialState.pressed)) {
@@ -204,7 +204,7 @@ class _MaterialStateColor extends MaterialStateColor {
 /// ```dart
 /// Widget build(BuildContext context) {
 ///   return ListTile(
-///     title: Text('Disabled ListTile'),
+///     title: const Text('Disabled ListTile'),
 ///     enabled: false,
 ///     mouseCursor: ListTileCursor(),
 ///   );
@@ -304,7 +304,7 @@ class _EnabledAndDisabledMouseCursor extends MaterialStateMouseCursor {
 ///   @override
 ///   BorderSide? resolve(Set<MaterialState> states) {
 ///     if (states.contains(MaterialState.selected)) {
-///       return BorderSide(
+///       return const BorderSide(
 ///         width: 1,
 ///         color: Colors.red,
 ///       );
@@ -317,9 +317,10 @@ class _EnabledAndDisabledMouseCursor extends MaterialStateMouseCursor {
 /// ```dart
 /// bool isSelected = true;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return FilterChip(
-///     label: Text('Select chip'),
+///     label: const Text('Select chip'),
 ///     selected: isSelected,
 ///     onSelected: (bool value) {
 ///       setState(() {
@@ -364,7 +365,7 @@ abstract class MaterialStateBorderSide extends BorderSide implements MaterialSta
 ///   @override
 ///   OutlinedBorder? resolve(Set<MaterialState> states) {
 ///     if (states.contains(MaterialState.selected)) {
-///       return RoundedRectangleBorder();
+///       return const RoundedRectangleBorder();
 ///     }
 ///     return null;  // Defer to default value on the theme or widget.
 ///   }
@@ -374,9 +375,10 @@ abstract class MaterialStateBorderSide extends BorderSide implements MaterialSta
 /// ```dart
 /// bool isSelected = true;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return FilterChip(
-///     label: Text('Select chip'),
+///     label: const Text('Select chip'),
 ///     selected: isSelected,
 ///     onSelected: (bool value) {
 ///       setState(() {
@@ -451,7 +453,7 @@ abstract class MaterialStateOutlinedBorder extends OutlinedBorder implements Mat
 ///       foregroundColor: MaterialStateProperty.resolveWith(getColor),
 ///     ),
 ///     onPressed: () {},
-///     child: Text('TextButton'),
+///     child: const Text('TextButton'),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/material/navigation_rail.dart

@@ -58,7 +58,7 @@ import 'theme.dart';
 ///              });
 ///            },
 ///            labelType: NavigationRailLabelType.selected,
-///            destinations: [
+///            destinations: const <NavigationRailDestination>[
 ///              NavigationRailDestination(
 ///                icon: Icon(Icons.favorite_border),
 ///                selectedIcon: Icon(Icons.favorite),
@@ -76,7 +76,7 @@ import 'theme.dart';
 ///              ),
 ///            ],
 ///          ),
-///          VerticalDivider(thickness: 1, width: 1),
+///          const VerticalDivider(thickness: 1, width: 1),
 ///          // This is the main content.
 ///          Expanded(
 ///            child: Center(
@@ -329,7 +329,7 @@ class NavigationRail extends StatefulWidget {
   /// This can be used to synchronize animations in the [leading] or [trailing]
   /// widget, such as an animated menu or a [FloatingActionButton] animation.
   ///
-  /// {@tool snippet}
+  /// {@tool dartpad --template=stateless_widget_material}
   ///
   /// This example shows how to use this animation to create a
   /// [FloatingActionButton] that animates itself between the normal and
@@ -338,10 +338,11 @@ class NavigationRail extends StatefulWidget {
   /// An instance of `ExtendableFab` would be created for
   /// [NavigationRail.leading].
   ///
-  /// ```dart
+  /// ```dart dartImports
   /// import 'dart:ui';
+  /// ```
   ///
-  /// @override
+  /// ```dart
   /// Widget build(BuildContext context) {
   ///   final Animation<double> animation = NavigationRail.extendedAnimation(context);
   ///   return AnimatedBuilder(
@@ -355,7 +356,7 @@ class NavigationRail extends StatefulWidget {
   ///         ),
   ///         child: animation.value == 0
   ///           ? FloatingActionButton(
-  ///               child: Icon(Icons.add),
+  ///               child: const Icon(Icons.add),
   ///               onPressed: () {},
   ///             )
   ///           : Align(
@@ -364,8 +365,8 @@ class NavigationRail extends StatefulWidget {
   ///               child: Padding(
   ///                 padding: const EdgeInsetsDirectional.only(start: 8),
   ///                 child: FloatingActionButton.extended(
-  ///                   icon: Icon(Icons.add),
+  ///                   icon: const Icon(Icons.add),
-  ///                   label: Text('CREATE'),
+  ///                   label: const Text('CREATE'),
   ///                   onPressed: () {},
   ///                 ),
   ///               ),

packages/flutter/lib/src/material/popup_menu.dart

@@ -22,8 +22,8 @@ import 'tooltip.dart';
 
 // Examples can assume:
 // enum Commands { heroAndScholar, hurricaneCame }
-// dynamic _heroAndScholar;
+// late bool _heroAndScholar;
-// dynamic _selection;
+// late dynamic _selection;
 // late BuildContext context;
 // void setState(VoidCallback fn) { }
 

packages/flutter/lib/src/material/progress_indicator.dart

@@ -267,14 +267,15 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
 ///   super.dispose();
 /// }
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     body: Padding(
 ///       padding: const EdgeInsets.all(20.0),
 ///       child: Column(
 ///         mainAxisAlignment: MainAxisAlignment.spaceEvenly,
-///         children: [
+///         children: <Widget>[
-///           Text(
+///           const Text(
 ///             'Linear progress indicator with a fixed color',
 ///             style: const TextStyle(fontSize: 20),
 ///           ),
@@ -505,13 +506,14 @@ class _CircularProgressIndicatorPainter extends CustomPainter {
 ///   super.dispose();
 /// }
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     body: Padding(
 ///       padding: const EdgeInsets.all(20.0),
 ///       child: Column(
 ///         mainAxisAlignment: MainAxisAlignment.spaceEvenly,
-///         children: [
+///         children: <Widget>[
 ///           Text(
 ///             'Linear progress indicator with a fixed color',
 ///             style: Theme.of(context).textTheme.headline6,

packages/flutter/lib/src/material/radio.dart

@@ -52,12 +52,13 @@ const double _kInnerRadius = 4.5;
 /// ```dart
 /// SingingCharacter? _character = SingingCharacter.lafayette;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Column(
 ///     children: <Widget>[
 ///       ListTile(
 ///         title: const Text('Lafayette'),
-///         leading: Radio(
+///         leading: Radio<SingingCharacter>(
 ///           value: SingingCharacter.lafayette,
 ///           groupValue: _character,
 ///           onChanged: (SingingCharacter? value) {
@@ -67,7 +68,7 @@ const double _kInnerRadius = 4.5;
 ///       ),
 ///       ListTile(
 ///         title: const Text('Thomas Jefferson'),
-///         leading: Radio(
+///         leading: Radio<SingingCharacter>(
 ///           value: SingingCharacter.jefferson,
 ///           groupValue: _character,
 ///           onChanged: (SingingCharacter? value) {
@@ -218,7 +219,7 @@ class Radio<T> extends StatefulWidget {
   /// Widget build(BuildContext context) {
   ///   return Scaffold(
   ///     body: ListView.builder(
-  ///       itemBuilder: (context, index) {
+  ///       itemBuilder: (BuildContext context, int index) {
   ///         return Row(
   ///           mainAxisSize: MainAxisSize.min,
   ///           crossAxisAlignment: CrossAxisAlignment.center,

packages/flutter/lib/src/material/radio_list_tile.dart

@@ -108,12 +108,13 @@ import 'theme_data.dart';
 /// ```dart preamble
 /// class LinkedLabelRadio extends StatelessWidget {
 ///   const LinkedLabelRadio({
+///     Key? key,
 ///     required this.label,
 ///     required this.padding,
 ///     required this.groupValue,
 ///     required this.value,
 ///     required this.onChanged,
-///   });
+///   }) :super(key: key);
 ///
 ///   final String label;
 ///   final EdgeInsets padding;
@@ -137,7 +138,7 @@ import 'theme_data.dart';
 ///           RichText(
 ///             text: TextSpan(
 ///               text: label,
-///               style: TextStyle(
+///               style: const TextStyle(
 ///                 color: Colors.blueAccent,
 ///                 decoration: TextDecoration.underline,
 ///               ),
@@ -164,7 +165,7 @@ import 'theme_data.dart';
 ///       children: <Widget>[
 ///         LinkedLabelRadio(
 ///           label: 'First tappable label text',
-///           padding: EdgeInsets.symmetric(horizontal: 5.0),
+///           padding: const EdgeInsets.symmetric(horizontal: 5.0),
 ///           value: true,
 ///           groupValue: _isRadioSelected,
 ///           onChanged: (bool newValue) {
@@ -175,7 +176,7 @@ import 'theme_data.dart';
 ///         ),
 ///         LinkedLabelRadio(
 ///           label: 'Second tappable label text',
-///           padding: EdgeInsets.symmetric(horizontal: 5.0),
+///           padding: const EdgeInsets.symmetric(horizontal: 5.0),
 ///           value: false,
 ///           groupValue: _isRadioSelected,
 ///           onChanged: (bool newValue) {
@@ -208,12 +209,13 @@ import 'theme_data.dart';
 /// ```dart preamble
 /// class LabeledRadio extends StatelessWidget {
 ///   const LabeledRadio({
+///     Key? key,
 ///     required this.label,
 ///     required this.padding,
 ///     required this.groupValue,
 ///     required this.value,
 ///     required this.onChanged,
-///   });
+///   }) : super(key: key);
 ///
 ///   final String label;
 ///   final EdgeInsets padding;
@@ -225,8 +227,9 @@ import 'theme_data.dart';
 ///   Widget build(BuildContext context) {
 ///     return InkWell(
 ///       onTap: () {
-///         if (value != groupValue)
+///         if (value != groupValue) {
 ///           onChanged(value);
+///         }
 ///       },
 ///       child: Padding(
 ///         padding: padding,
@@ -406,7 +409,7 @@ class RadioListTile<T> extends StatelessWidget {
   /// Widget build(BuildContext context) {
   ///   return Scaffold(
   ///     body: ListView.builder(
-  ///       itemBuilder: (context, index) {
+  ///       itemBuilder: (BuildContext context, int index) {
   ///         return RadioListTile<int>(
   ///           value: index,
   ///           groupValue: groupValue,

packages/flutter/lib/src/material/range_slider.dart

@@ -18,8 +18,8 @@ import 'slider_theme.dart';
 import 'theme.dart';
 
 // Examples can assume:
-// RangeValues _rangeValues = RangeValues(0.3, 0.7);
+// RangeValues _rangeValues = const RangeValues(0.3, 0.7);
-// RangeValues _dollarsRange = RangeValues(50, 100);
+// RangeValues _dollarsRange = const RangeValues(50, 100);
 // void setState(VoidCallback fn) { }
 
 /// [RangeSlider] uses this callback to paint the value indicator on the overlay.

packages/flutter/lib/src/material/reorderable_list.dart

@@ -34,10 +34,11 @@ import 'theme.dart';
 /// ```dart
 /// final List<int> _items = List<int>.generate(50, (int index) => index);
 ///
+/// @override
 /// Widget build(BuildContext context){
 ///   final ColorScheme colorScheme = Theme.of(context).colorScheme;
-///   final oddItemColor = colorScheme.primary.withOpacity(0.05);
+///   final Color oddItemColor = colorScheme.primary.withOpacity(0.05);
-///   final evenItemColor = colorScheme.primary.withOpacity(0.15);
+///   final Color evenItemColor = colorScheme.primary.withOpacity(0.15);
 ///
 ///   return ReorderableListView(
 ///     padding: const EdgeInsets.symmetric(horizontal: 40),
@@ -186,10 +187,11 @@ class ReorderableListView extends StatefulWidget {
   /// ```dart
   /// final List<int> _items = List<int>.generate(50, (int index) => index);
   ///
+  /// @override
   /// Widget build(BuildContext context){
   ///   final ColorScheme colorScheme = Theme.of(context).colorScheme;
-  ///   final oddItemColor = colorScheme.primary.withOpacity(0.05);
+  ///   final Color oddItemColor = colorScheme.primary.withOpacity(0.05);
-  ///   final evenItemColor = colorScheme.primary.withOpacity(0.15);
+  ///   final Color evenItemColor = colorScheme.primary.withOpacity(0.15);
   ///
   ///   return ReorderableListView(
   ///     buildDefaultDragHandles: false,
@@ -217,7 +219,7 @@ class ReorderableListView extends StatefulWidget {
   ///           ),
   ///         ),
   ///     ],
-  ///     onReorder: (oldIndex, newIndex) {
+  ///     onReorder: (int oldIndex, int newIndex) {
   ///       setState(() {
   ///         if (oldIndex < newIndex) {
   ///           newIndex -= 1;

packages/flutter/lib/src/material/scaffold.dart

@@ -152,9 +152,11 @@ class ScaffoldMessenger extends StatefulWidget {
   /// import 'package:flutter/material.dart';
   /// ```
   /// ```dart
-  /// void main() => runApp(MyApp());
+  /// void main() => runApp(const MyApp());
   ///
   /// class MyApp extends StatefulWidget {
+  ///   const MyApp({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///  _MyAppState createState() => _MyAppState();
   /// }
@@ -179,12 +181,12 @@ class ScaffoldMessenger extends StatefulWidget {
   ///     return MaterialApp(
   ///       scaffoldMessengerKey: _scaffoldMessengerKey,
   ///       home: Scaffold(
-  ///         appBar: AppBar(title: Text('ScaffoldMessenger Demo')),
+  ///         appBar: AppBar(title: const Text('ScaffoldMessenger Demo')),
   ///         body: Center(
   ///           child: Column(
   ///             mainAxisAlignment: MainAxisAlignment.center,
   ///             children: <Widget>[
-  ///               Text(
+  ///               const Text(
   ///                 'You have pushed the button this many times:',
   ///               ),
   ///               Text(
@@ -197,7 +199,7 @@ class ScaffoldMessenger extends StatefulWidget {
   ///         floatingActionButton: FloatingActionButton(
   ///           onPressed: _incrementCounter,
   ///           tooltip: 'Increment',
-  ///           child: Icon(Icons.add),
+  ///           child: const Icon(Icons.add),
   ///         ),
   ///       ),
   ///     );
@@ -1291,6 +1293,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// ```dart
 /// int _count = 0;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     appBar: AppBar(
@@ -1320,6 +1323,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// ```dart
 /// int _count = 0;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     appBar: AppBar(
@@ -1352,6 +1356,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// ```dart
 /// int _count = 0;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     appBar: AppBar(
@@ -1369,7 +1374,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 ///         _count++;
 ///       }),
 ///       tooltip: 'Increment Counter',
-///       child: Icon(Icons.add),
+///       child: const Icon(Icons.add),
 ///     ),
 ///     floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
 ///   );
@@ -1665,11 +1670,11 @@ class Scaffold extends StatefulWidget {
   /// Widget build(BuildContext context) {
   ///   return Scaffold(
   ///     key: _scaffoldKey,
-  ///     appBar: AppBar(title: Text('Drawer Demo')),
+  ///     appBar: AppBar(title: const Text('Drawer Demo')),
   ///     body: Center(
   ///       child: ElevatedButton(
   ///         onPressed: _openEndDrawer,
-  ///         child: Text('Open End Drawer'),
+  ///         child: const Text('Open End Drawer'),
   ///       ),
   ///     ),
   ///     endDrawer: Drawer(
@@ -1822,11 +1827,13 @@ class Scaffold extends StatefulWidget {
   /// ```
   ///
   /// ```dart main
-  /// void main() => runApp(MyApp());
+  /// void main() => runApp(const MyApp());
   /// ```
   ///
   /// ```dart preamble
   /// class MyApp extends StatelessWidget {
+  ///   const MyApp({Key? key}) : super(key: key);
+  ///
   ///   // This widget is the root of your application.
   ///   @override
   ///   Widget build(BuildContext context) {
@@ -1836,7 +1843,7 @@ class Scaffold extends StatefulWidget {
   ///         primarySwatch: Colors.blue,
   ///       ),
   ///       home: Scaffold(
-  ///         body: MyScaffoldBody(),
+  ///         body: const MyScaffoldBody(),
   ///         appBar: AppBar(title: const Text('Scaffold.of Example')),
   ///       ),
   ///       color: Colors.white,
@@ -1847,6 +1854,8 @@ class Scaffold extends StatefulWidget {
   ///
   /// ```dart
   /// class MyScaffoldBody extends StatelessWidget {
+  ///   const MyScaffoldBody({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///   Widget build(BuildContext context) {
   ///     return Center(
@@ -2213,8 +2222,8 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin, Resto
   ///     return OutlinedButton(
   ///       onPressed: () {
   ///         ScaffoldMessenger.of(context).showSnackBar(
-  ///           SnackBar(
+  ///           const SnackBar(
-  ///             content: const Text('A SnackBar has been shown.'),
+  ///             content: Text('A SnackBar has been shown.'),
   ///           ),
   ///         );
   ///       },

packages/flutter/lib/src/material/search.dart

@@ -101,7 +101,7 @@ abstract class SearchDelegate<T> {
   ///
   /// {@tool snippet}
   /// ```dart
-  /// class CustomSearchHintDelegate extends SearchDelegate {
+  /// class CustomSearchHintDelegate extends SearchDelegate<String> {
   ///   CustomSearchHintDelegate({
   ///     required String hintText,
   ///   }) : super(
@@ -111,22 +111,23 @@ abstract class SearchDelegate<T> {
   ///   );
   ///
   ///   @override
-  ///   Widget buildLeading(BuildContext context) => Text("leading");
+  ///   Widget buildLeading(BuildContext context) => const Text('leading');
   ///
+  ///   @override
   ///   PreferredSizeWidget buildBottom(BuildContext context) {
-  ///     return PreferredSize(
+  ///     return const PreferredSize(
   ///        preferredSize: Size.fromHeight(56.0),
-  ///        child: Text("bottom"));
+  ///        child: Text('bottom'));
   ///   }
   ///
   ///   @override
-  ///   Widget buildSuggestions(BuildContext context) => Text("suggestions");
+  ///   Widget buildSuggestions(BuildContext context) => const Text('suggestions');
   ///
   ///   @override
-  ///   Widget buildResults(BuildContext context) => Text('results');
+  ///   Widget buildResults(BuildContext context) => const Text('results');
   ///
   ///   @override
-  ///   List<Widget> buildActions(BuildContext context) => [];
+  ///   List<Widget> buildActions(BuildContext context) => <Widget>[];
   /// }
   /// ```
   /// {@end-tool}

packages/flutter/lib/src/material/selectable_text.dart

@@ -125,7 +125,7 @@ class _SelectableTextSelectionGestureDetectorBuilder extends TextSelectionGestur
 /// {@tool snippet}
 ///
 /// ```dart
-/// SelectableText(
+/// const SelectableText(
 ///   'Hello! How are you?',
 ///   textAlign: TextAlign.center,
 ///   style: TextStyle(fontWeight: FontWeight.bold),

packages/flutter/lib/src/material/slider_theme.dart

@@ -84,6 +84,8 @@ class SliderTheme extends InheritedTheme {
   ///
   /// ```dart
   /// class Launch extends StatefulWidget {
+  ///   const Launch({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///   State createState() => LaunchState();
   /// }
@@ -244,6 +246,8 @@ class SliderThemeData with Diagnosticable {
   ///
   /// ```dart
   /// class Blissful extends StatefulWidget {
+  ///   const Blissful({Key? key}) : super(key: key);
+  ///
   ///   @override
   ///   State createState() => BlissfulState();
   /// }

packages/flutter/lib/src/material/snack_bar.dart

@@ -165,13 +165,13 @@ class _SnackBarActionState extends State<SnackBarAction> {
 /// ```dart
 /// Widget build(BuildContext context) {
 ///   return ElevatedButton(
-///     child: Text("Show Snackbar"),
+///     child: const Text('Show Snackbar'),
 ///     onPressed: () {
 ///       ScaffoldMessenger.of(context).showSnackBar(
 ///         SnackBar(
-///           content: Text("Awesome Snackbar!"),
+///           content: const Text('Awesome Snackbar!'),
 ///           action: SnackBarAction(
-///             label: "Action",
+///             label: 'Action',
 ///             onPressed: () {
 ///               // Code to execute.
 ///             },
@@ -193,21 +193,22 @@ class _SnackBarActionState extends State<SnackBarAction> {
 /// ```dart
 /// Widget build(BuildContext context) {
 ///   return ElevatedButton(
-///     child: Text("Show Snackbar"),
+///     child: const Text('Show Snackbar'),
 ///     onPressed: () {
 ///       ScaffoldMessenger.of(context).showSnackBar(
 ///         SnackBar(
 ///           action: SnackBarAction(
-///             label: "Action",
+///             label: 'Action',
 ///             onPressed: () {
 ///               // Code to execute.
 ///             },
 ///           ),
-///           content: Text("Awesome SnackBar!"),
+///           content: const Text('Awesome SnackBar!'),
-///           duration: Duration(milliseconds: 1500),
+///           duration: const Duration(milliseconds: 1500),
 ///           width: 280.0, // Width of the SnackBar.
-///           padding: EdgeInsets.symmetric(
+///           padding: const EdgeInsets.symmetric(
-///             horizontal: 8.0), // Inner padding for SnackBar content.
+///             horizontal: 8.0,  // Inner padding for SnackBar content.
+///           ),
 ///           behavior: SnackBarBehavior.floating,
 ///           shape: RoundedRectangleBorder(
 ///             borderRadius: BorderRadius.circular(10.0),

packages/flutter/lib/src/material/stepper.dart

@@ -126,29 +126,33 @@ class Step {
 ///
 /// ```dart
 /// int _index = 0;
+///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Stepper(
 ///     currentStep: _index,
 ///     onStepCancel: () {
-///       if (_index > 0)
+///       if (_index > 0) {
 ///         setState(() { _index -= 1; });
+///       }
 ///     },
 ///     onStepContinue: () {
-///       if (_index <= 0)
+///       if (_index <= 0) {
 ///         setState(() { _index += 1; });
+///       }
 ///     },
-///     onStepTapped: (index) {
+///     onStepTapped: (int index) {
 ///       setState(() { _index = index; });
 ///     },
 ///     steps: <Step>[
 ///       Step(
-///         title: Text('Step 1 title'),
+///         title: const Text('Step 1 title'),
 ///         content: Container(
 ///           alignment: Alignment.centerLeft,
-///           child: Text('Content for Step 1')
+///           child: const Text('Content for Step 1')
 ///         ),
 ///       ),
-///       Step(
+///       const Step(
 ///         title: Text('Step 2 title'),
 ///         content: Text('Content for Step 2'),
 ///       ),

packages/flutter/lib/src/material/switch_list_tile.dart

@@ -102,11 +102,12 @@ enum _SwitchListTileType { material, adaptive }
 /// ```dart preamble
 /// class LinkedLabelSwitch extends StatelessWidget {
 ///   const LinkedLabelSwitch({
+///     Key? key,
 ///     required this.label,
 ///     required this.padding,
 ///     required this.value,
 ///     required this.onChanged,
-///   });
+///   }) : super(key: key);
 ///
 ///   final String label;
 ///   final EdgeInsets padding;
@@ -123,7 +124,7 @@ enum _SwitchListTileType { material, adaptive }
 ///             child: RichText(
 ///               text: TextSpan(
 ///                 text: label,
-///                 style: TextStyle(
+///                 style: const TextStyle(
 ///                   color: Colors.blueAccent,
 ///                   decoration: TextDecoration.underline,
 ///                 ),
@@ -182,11 +183,12 @@ enum _SwitchListTileType { material, adaptive }
 /// ```dart preamble
 /// class LabeledSwitch extends StatelessWidget {
 ///   const LabeledSwitch({
+///     Key? key,
 ///     required this.label,
 ///     required this.padding,
 ///     required this.value,
 ///     required this.onChanged,
-///   });
+///   }) : super(key: key);
 ///
 ///   final String label;
 ///   final EdgeInsets padding;

packages/flutter/lib/src/material/tab_controller.dart

@@ -38,7 +38,7 @@ import 'constants.dart';
 /// }
 ///
 /// class _MyTabbedPageState extends State<MyTabbedPage> with SingleTickerProviderStateMixin {
-///   final List<Tab> myTabs = <Tab>[
+///   static const List<Tab> myTabs = <Tab>[
 ///     Tab(text: 'LEFT'),
 ///     Tab(text: 'RIGHT'),
 ///   ];
@@ -90,7 +90,7 @@ import 'constants.dart';
 /// when using [DefaultTabController].
 ///
 /// ```dart preamble
-/// final List<Tab> tabs = <Tab>[
+/// const List<Tab> tabs = <Tab>[
 ///   Tab(text: 'Zeroth'),
 ///   Tab(text: 'First'),
 ///   Tab(text: 'Second'),
@@ -114,7 +114,7 @@ import 'constants.dart';
 ///         });
 ///         return Scaffold(
 ///           appBar: AppBar(
-///             bottom: TabBar(
+///             bottom: const TabBar(
 ///               tabs: tabs,
 ///             ),
 ///           ),

packages/flutter/lib/src/material/tabs.dart

@@ -579,8 +579,8 @@ class _TabBarScrollController extends ScrollController {
 ///      length: 3,
 ///      child: Scaffold(
 ///        appBar: AppBar(
-///          title: Text('TabBar Widget'),
+///          title: const Text('TabBar Widget'),
-///          bottom: TabBar(
+///          bottom: const TabBar(
 ///            tabs: <Widget>[
 ///              Tab(
 ///                icon: Icon(Icons.cloud_outlined),
@@ -594,7 +594,7 @@ class _TabBarScrollController extends ScrollController {
 ///            ],
 ///          ),
 ///        ),
-///        body: TabBarView(
+///        body: const TabBarView(
 ///          children: <Widget>[
 ///            Center(
 ///              child: Text('It\'s cloudy here'),
@@ -628,13 +628,14 @@ class _TabBarScrollController extends ScrollController {
 ///    _tabController = TabController(length: 3, vsync: this);
 ///  }
 ///
+///  @override
 ///  Widget build(BuildContext context) {
 ///    return Scaffold(
 ///      appBar: AppBar(
-///        title: Text('TabBar Widget'),
+///        title: const Text('TabBar Widget'),
 ///        bottom: TabBar(
 ///          controller: _tabController,
-///          tabs: <Widget>[
+///          tabs: const <Widget>[
 ///            Tab(
 ///              icon: Icon(Icons.cloud_outlined),
 ///            ),
@@ -649,7 +650,7 @@ class _TabBarScrollController extends ScrollController {
 ///      ),
 ///      body: TabBarView(
 ///        controller: _tabController,
-///        children: <Widget>[
+///        children: const <Widget>[
 ///          Center(
 ///            child: Text('It\'s cloudy here'),
 ///          ),

packages/flutter/lib/src/material/text_button.dart

@@ -64,7 +64,7 @@ import 'theme_data.dart';
 ///       children: <Widget>[
 ///         TextButton(
 ///            style: TextButton.styleFrom(
-///              textStyle: TextStyle(fontSize: 20),
+///              textStyle: const TextStyle(fontSize: 20),
 ///            ),
 ///            onPressed: null,
 ///            child: const Text('Disabled'),
@@ -72,7 +72,7 @@ import 'theme_data.dart';
 ///         const SizedBox(height: 30),
 ///         TextButton(
 ///           style: TextButton.styleFrom(
-///             textStyle: TextStyle(fontSize: 20),
+///             textStyle: const TextStyle(fontSize: 20),
 ///           ),
 ///           onPressed: () {},
 ///           child: const Text('Enabled'),
@@ -99,7 +99,7 @@ import 'theme_data.dart';
 ///                 style: TextButton.styleFrom(
 ///                   padding: const EdgeInsets.all(16.0),
 ///                   primary: Colors.white,
-///                   textStyle: TextStyle(fontSize: 20),
+///                   textStyle: const TextStyle(fontSize: 20),
 ///                 ),
 ///                 onPressed: () {},
 ///                  child: const Text('Gradient'),

packages/flutter/lib/src/material/text_field.dart

@@ -181,7 +181,7 @@ class _TextFieldSelectionGestureDetectorBuilder extends TextSelectionGestureDete
 /// ![](https://flutter.github.io/assets-for-api-docs/assets/material/text_field.png)
 ///
 /// ```dart
-/// TextField(
+/// const TextField(
 ///   obscureText: true,
 ///   decoration: InputDecoration(
 ///     border: OutlineInputBorder(),
@@ -205,16 +205,19 @@ class _TextFieldSelectionGestureDetectorBuilder extends TextSelectionGestureDete
 /// ```dart
 /// late TextEditingController _controller;
 ///
+/// @override
 /// void initState() {
 ///   super.initState();
 ///   _controller = TextEditingController();
 /// }
 ///
+/// @override
 /// void dispose() {
 ///   _controller.dispose();
 ///   super.dispose();
 /// }
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     body: Center(

packages/flutter/lib/src/material/text_form_field.dart

@@ -76,6 +76,7 @@ export 'package:flutter/services.dart' show SmartQuotesType, SmartDashesType;
 /// ```
 ///
 /// ```dart
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Material(
 ///     child: Center(

packages/flutter/lib/src/material/text_selection_theme.dart

@@ -118,7 +118,7 @@ class TextSelectionThemeData with Diagnosticable {
 /// color with light blue selection handles to the child text field.
 ///
 /// ```dart
-/// TextSelectionTheme(
+/// const TextSelectionTheme(
 ///   data: TextSelectionThemeData(
 ///     cursorColor: Colors.blue,
 ///     selectionHandleColor: Colors.lightBlue,

packages/flutter/lib/src/material/text_theme.dart

@@ -353,7 +353,7 @@ class TextTheme with Diagnosticable {
   /// /// A Widget that sets the ambient theme's title text color for its
   /// /// descendants, while leaving other ambient theme attributes alone.
   /// class TitleColorThemeCopy extends StatelessWidget {
-  ///   TitleColorThemeCopy({Key? key, required this.child, required this.titleColor}) : super(key: key);
+  ///   const TitleColorThemeCopy({Key? key, required this.child, required this.titleColor}) : super(key: key);
   ///
   ///   final Color titleColor;
   ///   final Widget child;
@@ -494,7 +494,7 @@ class TextTheme with Diagnosticable {
   /// /// A Widget that sets the ambient theme's title text color for its
   /// /// descendants, while leaving other ambient theme attributes alone.
   /// class TitleColorTheme extends StatelessWidget {
-  ///   TitleColorTheme({Key? key, required this.child, required this.titleColor}) : super(key: key);
+  ///   const TitleColorTheme({Key? key, required this.child, required this.titleColor}) : super(key: key);
   ///
   ///   final Color titleColor;
   ///   final Widget child;
@@ -507,7 +507,7 @@ class TextTheme with Diagnosticable {
   ///     // set the title, but everything else would be null. This isn't very
   ///     // useful, so merge it with the existing theme to keep all of the
   ///     // preexisting definitions for the other styles.
-  ///     TextTheme partialTheme = TextTheme(headline6: TextStyle(color: titleColor));
+  ///     final TextTheme partialTheme = TextTheme(headline6: TextStyle(color: titleColor));
   ///     theme = theme.copyWith(textTheme: theme.textTheme.merge(partialTheme));
   ///     return Theme(data: theme, child: child);
   ///   }

packages/flutter/lib/src/material/theme_data.dart

@@ -164,7 +164,7 @@ enum MaterialTapTargetSize {
 ///   theme: ThemeData(
 ///     primaryColor: Colors.blue,
 ///     accentColor: Colors.green,
-///     textTheme: TextTheme(bodyText2: TextStyle(color: Colors.purple)),
+///     textTheme: const TextTheme(bodyText2: TextStyle(color: Colors.purple)),
 ///   ),
 ///   home: Scaffold(
 ///     appBar: AppBar(
@@ -174,10 +174,8 @@ enum MaterialTapTargetSize {
 ///       child: const Icon(Icons.add),
 ///       onPressed: () {},
 ///     ),
-///     body: Center(
+///     body: const Center(
-///       child: Text(
+///       child: Text('Button pressed 0 times'),
-///         'Button pressed 0 times',
-///       ),
 ///     ),
 ///   ),
 /// )
@@ -722,8 +720,8 @@ class ThemeData with Diagnosticable {
   ///
   /// ```dart
   /// MaterialApp(
-  ///   theme: ThemeData.from(colorScheme: ColorScheme.light()),
+  ///   theme: ThemeData.from(colorScheme: const ColorScheme.light()),
-  ///   darkTheme: ThemeData.from(colorScheme: ColorScheme.dark()),
+  ///   darkTheme: ThemeData.from(colorScheme: const ColorScheme.dark()),
   /// )
   /// ```
   /// {@end-tool}

packages/flutter/lib/src/material/time.dart

@@ -31,7 +31,7 @@ enum DayPeriod {
 ///
 /// ```dart
 /// TimeOfDay now = TimeOfDay.now();
-/// TimeOfDay releaseTime = TimeOfDay(hour: 15, minute: 0); // 3:00pm
+/// const TimeOfDay releaseTime = TimeOfDay(hour: 15, minute: 0); // 3:00pm
 /// TimeOfDay roomBooked = TimeOfDay.fromDateTime(DateTime.parse('2018-10-20 16:30:04Z')); // 4:30pm
 /// ```
 /// {@end-tool}

packages/flutter/lib/src/material/time_picker.dart

@@ -2174,7 +2174,7 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
 /// ```dart
 /// Future<TimeOfDay?> selectedTime24Hour = showTimePicker(
 ///   context: context,
-///   initialTime: TimeOfDay(hour: 10, minute: 47),
+///   initialTime: const TimeOfDay(hour: 10, minute: 47),
 ///   builder: (BuildContext context, Widget? child) {
 ///     return MediaQuery(
 ///       data: MediaQuery.of(context).copyWith(alwaysUse24HourFormat: true),

packages/flutter/lib/src/material/tooltip.dart

@@ -38,9 +38,9 @@ import 'tooltip_theme.dart';
 ///
 /// ```dart
 /// Widget build(BuildContext context) {
-///   return Tooltip(
+///   return const Tooltip(
-///     message: "I am a Tooltip",
+///     message: 'I am a Tooltip',
-///     child: Text("Hover over the text to show a tooltip."),
+///     child: Text('Hover over the text to show a tooltip.'),
 ///   );
 /// }
 /// ```
@@ -63,20 +63,20 @@ import 'tooltip_theme.dart';
 /// ```dart
 /// Widget build(BuildContext context) {
 ///   return Tooltip(
-///     message: "I am a Tooltip",
+///     message: 'I am a Tooltip',
-///     child: Text("Tap this text and hold down to show a tooltip."),
+///     child: const Text('Tap this text and hold down to show a tooltip.'),
 ///     decoration: BoxDecoration(
 ///       borderRadius: BorderRadius.circular(25),
-///       gradient: LinearGradient(colors: [Colors.amber, Colors.red]),
+///       gradient: const LinearGradient(colors: <Color>[Colors.amber, Colors.red]),
 ///     ),
 ///     height: 50,
-///     padding: EdgeInsets.all(8.0),
+///     padding: const EdgeInsets.all(8.0),
 ///     preferBelow: false,
-///     textStyle: TextStyle(
+///     textStyle: const TextStyle(
 ///       fontSize: 24,
 ///     ),
-///     showDuration: Duration(seconds: 2),
+///     showDuration: const Duration(seconds: 2),
-///     waitDuration: Duration(seconds: 1),
+///     waitDuration: const Duration(seconds: 1),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/material/tooltip_theme.dart

@@ -207,7 +207,7 @@ class TooltipThemeData with Diagnosticable {
 ///     message: 'Example tooltip',
 ///     child: IconButton(
 ///       iconSize: 36.0,
-///       icon: Icon(Icons.touch_app),
+///       icon: const Icon(Icons.touch_app),
 ///       onPressed: () {},
 ///     ),
 ///   ),

packages/flutter/lib/src/painting/borders.dart

@@ -38,14 +38,14 @@ enum BorderStyle {
 ///
 /// ```dart
 /// Container(
-///   padding: EdgeInsets.all(8.0),
+///   padding: const EdgeInsets.all(8.0),
 ///   decoration: BoxDecoration(
 ///     border: Border(
 ///       top: BorderSide(width: 16.0, color: Colors.lightBlue.shade50),
 ///       bottom: BorderSide(width: 16.0, color: Colors.lightBlue.shade900),
 ///     ),
 ///   ),
-///   child: Text('Flutter in the sky', textAlign: TextAlign.center),
+///   child: const Text('Flutter in the sky', textAlign: TextAlign.center),
 /// )
 /// ```
 /// {@end-tool}

packages/flutter/lib/src/painting/box_border.dart

@@ -267,20 +267,20 @@ abstract class BoxBorder extends ShapeBorder {
 /// Container(
 ///   decoration: const BoxDecoration(
 ///     border: Border(
-///       top: BorderSide(width: 1.0, color: Color(0xFFFFFFFFFF)),
+///       top: BorderSide(width: 1.0, color: Color(0xFFFFFFFF)),
-///       left: BorderSide(width: 1.0, color: Color(0xFFFFFFFFFF)),
+///       left: BorderSide(width: 1.0, color: Color(0xFFFFFFFF)),
-///       right: BorderSide(width: 1.0, color: Color(0xFFFF000000)),
+///       right: BorderSide(width: 1.0, color: Color(0xFF000000)),
-///       bottom: BorderSide(width: 1.0, color: Color(0xFFFF000000)),
+///       bottom: BorderSide(width: 1.0, color: Color(0xFF000000)),
 ///     ),
 ///   ),
 ///   child: Container(
 ///     padding: const EdgeInsets.symmetric(horizontal: 20.0, vertical: 2.0),
 ///     decoration: const BoxDecoration(
 ///       border: Border(
-///         top: BorderSide(width: 1.0, color: Color(0xFFFFDFDFDF)),
+///         top: BorderSide(width: 1.0, color: Color(0xFFDFDFDF)),
-///         left: BorderSide(width: 1.0, color: Color(0xFFFFDFDFDF)),
+///         left: BorderSide(width: 1.0, color: Color(0xFFDFDFDF)),
-///         right: BorderSide(width: 1.0, color: Color(0xFFFF7F7F7F)),
+///         right: BorderSide(width: 1.0, color: Color(0xFF7F7F7F)),
-///         bottom: BorderSide(width: 1.0, color: Color(0xFFFF7F7F7F)),
+///         bottom: BorderSide(width: 1.0, color: Color(0xFF7F7F7F)),
 ///       ),
 ///       color: Color(0xFFBFBFBF),
 ///     ),

packages/flutter/lib/src/painting/gradient.dart

@@ -338,11 +338,11 @@ abstract class Gradient {
 /// ```dart
 ///  Widget build(BuildContext context) {
 ///    return Container(
-///      decoration: BoxDecoration(
+///      decoration: const BoxDecoration(
-///        gradient: LinearGradient(
+///        gradient: const LinearGradient(
 ///          begin: Alignment.topLeft,
 ///          end: Alignment(0.8, 0.0), // 10% of the width, so there are ten blinds.
-///          colors: [const Color(0xffee0000), const Color(0xffeeee00)], // red to yellow
+///          colors: const <Color>[Color(0xffee0000), Color(0xffeeee00)], // red to yellow
 ///          tileMode: TileMode.repeated, // repeats the gradient over the canvas
 ///        ),
 ///      ),
@@ -562,17 +562,17 @@ class LinearGradient extends Gradient {
 ///
 /// ```dart
 /// void paintSky(Canvas canvas, Rect rect) {
-///   var gradient = RadialGradient(
+///   const RadialGradient gradient = RadialGradient(
-///     center: const Alignment(0.7, -0.6), // near the top right
+///     center: Alignment(0.7, -0.6), // near the top right
 ///     radius: 0.2,
-///     colors: [
+///     colors: <Color>[
-///       const Color(0xFFFFFF00), // yellow sun
+///       Color(0xFFFFFF00), // yellow sun
-///       const Color(0xFF0099FF), // blue sky
+///       Color(0xFF0099FF), // blue sky
 ///     ],
-///     stops: [0.4, 1.0],
+///     stops: <double>[0.4, 1.0],
 ///   );
 ///   // rect is the area we are painting over
-///   var paint = Paint()
+///   final Paint paint = Paint()
 ///     ..shader = gradient.createShader(rect);
 ///   canvas.drawRect(rect, paint);
 /// }
@@ -810,19 +810,19 @@ class RadialGradient extends Gradient {
 ///
 /// ```dart
 /// Container(
-///   decoration: BoxDecoration(
+///   decoration: const BoxDecoration(
 ///     gradient: SweepGradient(
 ///       center: FractionalOffset.center,
 ///       startAngle: 0.0,
 ///       endAngle: math.pi * 2,
-///       colors: const <Color>[
+///       colors: <Color>[
 ///         Color(0xFF4285F4), // blue
 ///         Color(0xFF34A853), // green
 ///         Color(0xFFFBBC05), // yellow
 ///         Color(0xFFEA4335), // red
 ///         Color(0xFF4285F4), // blue again to seamlessly transition to the start
 ///       ],
-///       stops: const <double>[0.0, 0.25, 0.5, 0.75, 1.0],
+///       stops: <double>[0.0, 0.25, 0.5, 0.75, 1.0],
 ///     ),
 ///   )
 /// )
@@ -836,19 +836,19 @@ class RadialGradient extends Gradient {
 ///
 /// ```dart
 /// Container(
-///   decoration: BoxDecoration(
+///   decoration: const BoxDecoration(
 ///     gradient: SweepGradient(
 ///       center: FractionalOffset.center,
 ///       startAngle: 0.0,
 ///       endAngle: math.pi * 2,
-///       colors: const <Color>[
+///       colors: <Color>[
 ///         Color(0xFF4285F4), // blue
 ///         Color(0xFF34A853), // green
 ///         Color(0xFFFBBC05), // yellow
 ///         Color(0xFFEA4335), // red
 ///         Color(0xFF4285F4), // blue again to seamlessly transition to the start
 ///       ],
-///       stops: const <double>[0.0, 0.25, 0.5, 0.75, 1.0],
+///       stops: <double>[0.0, 0.25, 0.5, 0.75, 1.0],
 ///       transform: GradientRotation(math.pi/4),
 ///     ),
 ///   ),

packages/flutter/lib/src/painting/image_cache.dart

@@ -19,7 +19,7 @@ const int _kDefaultSizeBytes = 100 << 20; // 100 MiB
 /// MB. The maximum size can be adjusted using [maximumSize] and
 /// [maximumSizeBytes].
 ///
-/// The cache also holds a list of "live" references. An image is considered
+/// The cache also holds a list of 'live' references. An image is considered
 /// live if its [ImageStreamCompleter]'s listener count has never dropped to
 /// zero after adding at least one listener. The cache uses
 /// [ImageStreamCompleter.addOnLastListenerRemovedCallback] to determine when
@@ -28,7 +28,7 @@ const int _kDefaultSizeBytes = 100 << 20; // 100 MiB
 /// The [putIfAbsent] method is the main entry-point to the cache API. It
 /// returns the previously cached [ImageStreamCompleter] for the given key, if
 /// available; if not, it calls the given callback to obtain it first. In either
-/// case, the key is moved to the "most recently used" position.
+/// case, the key is moved to the 'most recently used' position.
 ///
 /// A caller can determine whether an image is already in the cache by using
 /// [containsKey], which will return true if the image is tracked by the cache
@@ -52,7 +52,7 @@ const int _kDefaultSizeBytes = 100 << 20; // 100 MiB
 /// class MyImageCache extends ImageCache {
 ///   @override
 ///   void clear() {
-///     print("Clearing cache!");
+///     print('Clearing cache!');
 ///     super.clear();
 ///   }
 /// }
@@ -65,10 +65,12 @@ const int _kDefaultSizeBytes = 100 << 20; // 100 MiB
 /// void main() {
 ///   // The constructor sets global variables.
 ///   MyWidgetsBinding();
-///   runApp(MyApp());
+///   runApp(const MyApp());
 /// }
 ///
 /// class MyApp extends StatelessWidget {
+///   const MyApp({Key? key}) : super(key: key);
+///
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return Container();
@@ -309,7 +311,7 @@ class ImageCache {
 
   /// Returns the previously cached [ImageStream] for the given key, if available;
   /// if not, calls the given callback to obtain it first. In either case, the
-  /// key is moved to the "most recently used" position.
+  /// key is moved to the 'most recently used' position.
   ///
   /// The arguments must not be null. The `loader` cannot return null.
   ///