Prepare to migrate API doc samples and snippets to null safety (#72040)

dev/bots/analyze-sample-code.dart

@@ -8,9 +8,11 @@
 //   bin/cache/dart-sdk/bin/dart dev/bots/analyze-sample-code.dart
 
 import 'dart:io';
+import 'dart:isolate';
 
 import 'package:args/args.dart';
 import 'package:path/path.dart' as path;
+import 'package:watcher/watcher.dart';
 
 final String _flutterRoot = path.dirname(path.dirname(path.dirname(path.fromUri(Platform.script))));
 final String _defaultFlutterPackage = path.join(_flutterRoot, 'packages', 'flutter', 'lib');
@@ -37,6 +39,11 @@ void main(List<String> arguments) {
     negatable: false,
     help: 'Print help for this command.',
   );
+  argParser.addOption(
+    'interactive',
+    abbr: 'i',
+    help: 'Analyzes the sample code in the specified file interactivly.',
+  );
 
   final ArgResults parsedArguments = argParser.parse(arguments);
 
@@ -69,15 +76,20 @@ void main(List<String> arguments) {
     }
     tempDirectory.createSync();
   }
-  try {
+
-    exitCode = SampleChecker(
+  if (parsedArguments['interactive'] != null) {
-      flutterPackage,
+    _runInteractive(tempDirectory, flutterPackage, parsedArguments['interactive'] as String);
-      tempDirectory: tempDirectory,
+  } else {
-      verbose: parsedArguments['verbose'] as bool,
+    try {
-    ).checkSamples();
+      exitCode = SampleChecker(
-  } on SampleCheckerException catch (e) {
+        flutterPackage,
-    stderr.write(e);
+        tempDirectory: tempDirectory,
-    exit(1);
+        verbose: parsedArguments['verbose'] as bool,
+      ).checkSamples();
+    } on SampleCheckerException catch (e) {
+      stderr.write(e);
+      exit(1);
+    }
   }
 }
 
@@ -141,6 +153,9 @@ class SampleChecker {
   /// A RegExp that matches a Dart constructor.
   static final RegExp _constructorRegExp = RegExp(r'(const\s+)?_*[A-Z][a-zA-Z0-9<>._]*\(');
 
+  /// A RegExp that matches a dart version specification in an example preamble.
+  static final RegExp _dartVersionRegExp = RegExp(r'\/\/ \/\/ @dart = ([0-9]+\.[0-9]+)');
+
   /// Whether or not to print verbose output.
   final bool verbose;
 
@@ -186,8 +201,8 @@ class SampleChecker {
   /// Computes the headers needed for each sample file.
   List<Line> get headers {
     return _headers ??= <String>[
-      '// @dart = 2.9',
       '// generated code',
+      '// ignore_for_file: unused_import',
       "import 'dart:async';",
       "import 'dart:convert';",
       "import 'dart:math' as math;",
@@ -211,7 +226,7 @@ class SampleChecker {
     try {
       final Map<String, Section> sections = <String, Section>{};
       final Map<String, Sample> snippets = <String, Sample>{};
-      _extractSamples(sections, snippets);
+      _extractSamples(_listDartFiles(_flutterPackage, recursive: true), sectionMap: sections, sampleMap: snippets);
       errors = _analyze(_tempDirectory, sections, snippets);
     } finally {
       if (errors.isNotEmpty) {
@@ -305,13 +320,13 @@ class SampleChecker {
     return outputFile;
   }
 
-  /// Extracts the samples from the Dart files in [_flutterPackage], writes them
+  /// Extracts the samples from the Dart files in [files], writes them
   /// to disk, and adds them to the appropriate [sectionMap] or [sampleMap].
-  void _extractSamples(Map<String, Section> sectionMap, Map<String, Sample> sampleMap) {
+  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>[];
 
-    for (final File file in _listDartFiles(_flutterPackage, recursive: true)) {
+    for (final File file in files) {
       final String relativeFilePath = path.relative(file.path, from: _flutterPackage.path);
       final List<String> sampleLines = file.readAsLinesSync();
       final List<Section> preambleSections = <Section>[];
@@ -323,6 +338,7 @@ class SampleChecker {
       bool inSnippet = false;
       // Whether or not we're in a '```dart' segment.
       bool inDart = false;
+      String dartVersionOverride;
       int lineNumber = 0;
       final List<String> block = <String>[];
       List<String> snippetArgs = <String>[];
@@ -353,10 +369,16 @@ class SampleChecker {
         } else if (inPreamble) {
           if (line.isEmpty) {
             inPreamble = false;
-            preambleSections.add(_processBlock(startLine, block));
+            // If there's only a dartVersionOverride in the preamble, don't add
+            // it as a section. The dartVersionOverride was processed below.
+            if (dartVersionOverride == null || block.isNotEmpty) {
+              preambleSections.add(_processBlock(startLine, block));
+            }
             block.clear();
           } else if (!line.startsWith('// ')) {
             throw SampleCheckerException('Unexpected content in sample code preamble.', file: relativeFilePath, line: lineNumber);
+          } else if (_dartVersionRegExp.hasMatch(line)) {
+            dartVersionOverride = line.substring(3);
           } else {
             block.add(line.substring(3));
           }
@@ -371,11 +393,8 @@ class SampleChecker {
             if (_codeBlockEndRegex.hasMatch(trimmedLine)) {
               inDart = false;
               final Section processed = _processBlock(startLine, block);
-              if (preambleSections.isEmpty) {
+              final Section combinedSection = preambleSections.isEmpty ? processed : Section.combine(preambleSections..add(processed));
-                sections.add(processed);
+              sections.add(combinedSection.copyWith(dartVersionOverride: dartVersionOverride));
-              } else {
-                sections.add(Section.combine(preambleSections..add(processed)));
-              }
               block.clear();
             } else if (trimmedLine == _dartDocPrefix) {
               block.add('');
@@ -434,14 +453,19 @@ class SampleChecker {
         }
       }
     }
-    print('Found ${sections.length} sample code sections.');
+    if (!silent)
+      print('Found ${sections.length} sample code sections.');
     for (final Section section in sections) {
-      sectionMap[_writeSection(section).path] = section;
+      final String path = _writeSection(section).path;
+      if (sectionMap != null)
+        sectionMap[path] = section;
     }
     for (final Sample sample in samples) {
       final File snippetFile = _writeSample(sample);
-      sample.contents = snippetFile.readAsLinesSync();
+      if (sampleMap != null) {
-      sampleMap[snippetFile.absolute.path] = sample;
+        sample.contents = snippetFile.readAsLinesSync();
+        sampleMap[snippetFile.absolute.path] = sample;
+      }
     }
   }
 
@@ -492,7 +516,7 @@ class SampleChecker {
     pubSpec.writeAsStringSync('''
 name: analyze_sample_code
 environment:
-  sdk: '>=2.10.0 <3.0.0'
+  sdk: ">=2.12.0-0 <3.0.0"
 dependencies:
   flutter:
     sdk: flutter
@@ -512,6 +536,7 @@ linter:
     final String sectionId = _createNameFromSource('snippet', section.start.filename, section.start.line);
     final File outputFile = File(path.join(_tempDirectory.path, '$sectionId.dart'))..createSync(recursive: true);
     final List<Line> mainContents = <Line>[
+      if (section.dartVersionOverride != null) Line(section.dartVersionOverride) else const Line(''),
       ...headers,
       const Line(''),
       Line('// From: ${section.start.filename}:${section.start.line}'),
@@ -582,7 +607,7 @@ linter:
       caseSensitive: false,
     );
     bool unknownAnalyzerErrors = false;
-    final int headerLength = headers.length + 2;
+    final int headerLength = headers.length + 3;
     for (final String error in errors) {
       final Match parts = errorPattern.matchAsPrefix(error);
       if (parts != null) {
@@ -803,7 +828,7 @@ class Line {
 
 /// A class to represent a section of sample code, marked by "{@tool snippet}...{@end-tool}".
 class Section {
-  const Section(this.code);
+  const Section(this.code, {this.dartVersionOverride});
   factory Section.combine(List<Section> sections) {
     final List<Line> code = sections
       .expand((Section section) => section.code)
@@ -846,6 +871,11 @@ class Section {
   }
   Line get start => code.firstWhere((Line line) => line.filename != null);
   final List<Line> code;
+  final String dartVersionOverride;
+
+  Section copyWith({String dartVersionOverride}) {
+    return Section(code, dartVersionOverride: dartVersionOverride);
+  }
 }
 
 /// A class to represent a sample in the dartdoc comments, marked by
@@ -908,3 +938,46 @@ class AnalysisError {
     }
   }
 }
+
+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()) {
+    throw 'Path ${file.absolute.path} does not exist.';
+  }
+  if (!path.isWithin(_flutterRoot, file.absolute.path)) {
+    throw 'Path ${file.absolute.path} is not within the flutter root: $_flutterRoot';
+  }
+
+  if (tempDir == null) {
+    tempDir = Directory.systemTemp.createTempSync('flutter_analyze_sample_code.');
+    ProcessSignal.sigint.watch().listen((_) {
+      print('Deleting temp files...');
+      tempDir.deleteSync(recursive: true);
+      exit(0);
+    });
+    print('Using temp dir ${tempDir.path}');
+  }
+
+  final SampleChecker checker = SampleChecker(flutterPackage, tempDirectory: tempDir)
+    .._createConfigurationFiles(tempDir)
+    .._extractSamples(<File>[file], silent: true);
+
+  await Isolate.spawn(_watcher, <dynamic>[checker, file]);
+
+  await Process.start(
+    _flutter,
+    <String>['--no-wrap', 'analyze', '--no-preamble', '--no-congratulate', '--watch', '.'],
+    workingDirectory: tempDir.absolute.path,
+    mode: ProcessStartMode.inheritStdio
+  );
+}
+
+void _watcher(List<dynamic> args) {
+  final File file = args.last as File;
+  final SampleChecker checker = args.first as SampleChecker;
+  Watcher(file.absolute.path).events.listen((_) {
+    checker._extractSamples(<File>[file], silent: true);
+  });
+}

dev/bots/test/analyze-sample-code-test-input/known_broken_documentation.dart

@@ -6,6 +6,7 @@
 // precise contents (including especially the comments) of this file.
 
 // Examples can assume:
+// // @dart = 2.9
 // bool _visible = true;
 // class _Text extends Text {
 //   const _Text(String text) : super(text);
@@ -34,7 +35,7 @@
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_material}
+/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
 /// Bla blabla blabla some [Text] when the `_blabla` blabla blabla is true, and
 /// blabla it when it is blabla:
 ///

dev/bots/test/analyze-sample-code-test-null-safe-input/known_broken_documentation.dart

@@ -0,0 +1,134 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This file is used by ../analyze-sample-code_test.dart, which depends on the
+// precise contents (including especially the comments) of this file.
+
+// Examples can assume:
+// bool _visible = true;
+// class _Text extends Text {
+//   const _Text(String text) : super(text);
+//   const _Text.__(String text) : super(text);
+// }
+
+/// A blabla that blabla its blabla blabla blabla.
+///
+/// Bla blabla blabla its blabla into an blabla blabla and then blabla the
+/// blabla back into the blabla blabla blabla.
+///
+/// Bla blabla of blabla blabla than 0.0 and 1.0, this blabla is blabla blabla
+/// blabla it blabla pirates blabla the blabla into of blabla blabla. Bla the
+/// blabla 0.0, the penzance blabla is blabla not blabla at all. Bla the blabla
+/// 1.0, the blabla is blabla blabla blabla an blabla blabla.
+///
+/// {@tool snippet}
+/// Bla blabla blabla some [Text] when the `_blabla` blabla blabla is true, and
+/// blabla it when it is blabla:
+///
+/// ```dart
+/// new Opacity(
+///   opacity: _visible ? 1.0 : 0.0,
+///   child: const Text('Poor wandering ones!'),
+/// )
+/// ```
+/// {@end-tool}
+///
+/// {@tool dartpad --template=stateless_widget_material}
+/// Bla blabla blabla some [Text] when the `_blabla` blabla blabla is true, and
+/// blabla it when it is blabla:
+///
+/// ```dart preamble
+/// bool _visible = true;
+/// final GlobalKey globalKey = GlobalKey();
+/// ```
+///
+/// ```dart
+/// Widget build(BuildContext context) {
+///   return Opacity(
+///     key: globalKey,
+///     opacity: _visible ? 1.0 : 0.0,
+///     child: const Text('Poor wandering ones!'),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// {@tool snippet}
+/// Bla blabla blabla some [Text] when the `_blabla` blabla blabla is true, and
+/// blabla finale blabla:
+///
+/// ```dart
+/// new Opacity(
+///   opacity: _visible ? 1.0 : 0.0,
+///   child: const Text('Poor wandering ones!'),
+/// )
+/// ```
+/// {@end-tool}
+///
+/// {@tool snippet}
+/// regular const constructor
+///
+/// ```dart
+/// const Text('Poor wandering ones!')
+/// ```
+/// {@end-tool}
+///
+/// {@tool snippet}
+/// const private constructor
+/// ```dart
+/// const             _Text('Poor wandering ones!')
+/// ```
+/// {@end-tool}
+///
+/// {@tool snippet}
+/// yet another const private constructor
+/// ```dart
+/// const        _Text.__('Poor wandering ones!')
+/// ```
+/// {@end-tool}
+///
+/// {@tool snippet}
+/// const variable
+///
+/// ```dart
+/// const text0 = Text('Poor wandering ones!');
+/// ```
+/// {@end-tool}
+///
+/// {@tool snippet}
+/// more const variables
+///
+/// ```dart
+/// const text1 = _Text('Poor wandering ones!');
+/// ```
+/// {@end-tool}
+///
+/// {@tool snippet}
+/// Snippet with null-safe syntax
+///
+/// ```dart
+/// final String? bar = 'Hello';
+/// final int foo = null;
+/// ```
+/// {@end-tool}
+///
+/// {@tool dartpad --template=stateless_widget_material}
+/// Dartpad with null-safe syntax
+///
+/// ```dart preamble
+/// bool? _visible = true;
+/// final GlobalKey globalKey = GlobalKey();
+/// ```
+///
+/// ```dart
+/// Widget build(BuildContext context) {
+///   final String title;
+///   return Opacity(
+///     key: globalKey,
+///     opacity: _visible! ? 1.0 : 0.0,
+///     child: Text(title),
+///   );
+/// }
+/// ```
+/// {@end-tool}

dev/bots/test/analyze-sample-code_test.dart

@@ -17,9 +17,9 @@ void main() {
       ..removeWhere((String line) => line.startsWith('Analyzer output:') || line.startsWith('Building flutter tool...'));
     expect(process.exitCode, isNot(equals(0)));
     expect(stderrLines, <String>[
-      'known_broken_documentation.dart:30:9: new Opacity(',
+      'known_broken_documentation.dart:31:9: new Opacity(',
       '>>> Unnecessary new keyword (unnecessary_new)',
-      'known_broken_documentation.dart:62:9: new Opacity(',
+      'known_broken_documentation.dart:63:9: new Opacity(',
       '>>> Unnecessary new keyword (unnecessary_new)',
       '',
       'Found 1 sample code errors.',
@@ -27,7 +27,36 @@ void main() {
     ]);
     expect(stdoutLines, <String>[
       'Found 7 sample code sections.',
-       'Starting analysis of code samples.',
+      'Starting analysis of code samples.',
+      '',
+    ]);
+  }, skip: Platform.isWindows);
+
+  test('analyze-sample-code null-safe', () {
+    final ProcessResult process = Process.runSync(
+      '../../bin/cache/dart-sdk/bin/dart',
+      <String>['analyze-sample-code.dart', 'test/analyze-sample-code-test-null-safe-input'],
+    );
+    final List<String> stdoutLines = process.stdout.toString().split('\n');
+    final List<String> stderrLines = process.stderr.toString().split('\n')
+      ..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:      child: Text(title),',
+      '>>> 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)',
+      'known_broken_documentation.dart:62:9: new Opacity(',
+      '>>> Unnecessary new keyword (unnecessary_new)',
+      '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)',
+      '',
+      'Found 2 sample code errors.',
+      ''
+    ]);
+    expect(stdoutLines, <String>[
+      'Found 8 sample code sections.',
+      'Starting analysis of code samples.',
       '',
     ]);
   }, skip: Platform.isWindows);

dev/snippets/config/skeletons/dartpad-sample.html

@@ -23,7 +23,7 @@
 <div class="snippet-container">
     <div class="snippet" id="longSnippet{{serial}}">
         {{description}}
-        <iframe class="snippet-dartpad" src="https://dartpad.dev/embed-flutter.html?split=60&run=true&sample_id={{id}}&sample_channel={{channel}}"></iframe>
+        <iframe class="snippet-dartpad" src="https://dartpad.dev/embed-flutter.html?split=60&run=true&null_safety=true&sample_id={{id}}&sample_channel={{channel}}"></iframe>
         <div class="snippet-description">To create a local project with this code sample, run:<br/>
             <span class="snippet-create-command">flutter create --sample={{id}} mysample</span>
         </div>

dev/snippets/config/templates/freeform.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/freeform_no_null_safety.tmpl

@@ -0,0 +1,12 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+{{code-imports}}
+
+{{code-main}}
+
+{{code-preamble}}
+
+{{code}}

dev/snippets/config/templates/stateful_widget.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -25,7 +24,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_cupertino.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -26,7 +25,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_cupertino_no_null_safety.tmpl

@@ -0,0 +1,38 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/cupertino.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoApp(
+      title: _title,
+      home: MyStatefulWidget(),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> {
+  {{code}}
+}

dev/snippets/config/templates/stateful_widget_cupertino_page_scaffold.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -29,7 +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);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_cupertino_page_scaffold_no_null_safety.tmpl

@@ -0,0 +1,41 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/cupertino.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoApp(
+      title: _title,
+      home: CupertinoPageScaffold(
+        navigationBar: CupertinoNavigationBar(middle: const Text(_title)),
+        child: MyStatefulWidget(),
+      ),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> {
+  {{code}}
+}

dev/snippets/config/templates/stateful_widget_cupertino_ticker.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -26,7 +25,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_cupertino_ticker_no_null_safety.tmpl

@@ -0,0 +1,39 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/cupertino.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoApp(
+      title: _title,
+      home: MyStatefulWidget(),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+/// AnimationControllers can be created with `vsync: this` because of TickerProviderStateMixin.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> with TickerProviderStateMixin {
+  {{code}}
+}

dev/snippets/config/templates/stateful_widget_material.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -26,7 +25,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_material_no_null_safety.tmpl

@@ -0,0 +1,38 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: MyStatefulWidget(),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> {
+  {{code}}
+}

dev/snippets/config/templates/stateful_widget_material_ticker.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -26,7 +25,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_material_ticker_no_null_safety.tmpl

@@ -0,0 +1,39 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: MyStatefulWidget(),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+/// AnimationControllers can be created with `vsync: this` because of TickerProviderStateMixin.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> with TickerProviderStateMixin {
+  {{code}}
+}

dev/snippets/config/templates/stateful_widget_no_null_safety.tmpl

@@ -0,0 +1,37 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/widgets.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return WidgetsApp(
+      title: 'Flutter Code Sample',
+      home: MyStatefulWidget(),
+      color: const Color(0xffffffff),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> {
+  {{code}}
+}

dev/snippets/config/templates/stateful_widget_restoration.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -27,9 +26,9 @@ 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);
+  MyStatefulWidget({Key? key, this.restorationId}) : super(key: key);
 
-  final String restorationId;
+  final String? restorationId;
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
@@ -41,7 +40,7 @@ class _MyStatefulWidgetState extends State<MyStatefulWidget> with RestorationMix
   // In this example, the restoration ID for the mixin is passed in through
   // the [StatefulWidget]'s constructor.
   @override
-  String get restorationId => widget.restorationId;
+  String? get restorationId => widget.restorationId;
 
 {{code}}
 }

dev/snippets/config/templates/stateful_widget_restoration_no_null_safety.tmpl

@@ -0,0 +1,47 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return WidgetsApp(
+      title: 'Flutter Code Sample',
+      home: Center(
+        child: MyStatefulWidget(restorationId: 'main'),
+      ),
+      color: const Color(0xffffffff),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key, this.restorationId}) : super(key: key);
+
+  final String restorationId;
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+/// RestorationProperty objects can be used because of RestorationMixin.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> with RestorationMixin {
+  // In this example, the restoration ID for the mixin is passed in through
+  // the [StatefulWidget]'s constructor.
+  @override
+  String get restorationId => widget.restorationId;
+
+{{code}}
+}

dev/snippets/config/templates/stateful_widget_scaffold.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -29,7 +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);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_scaffold_center.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -31,7 +30,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_scaffold_center_freeform_state.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -31,7 +30,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_scaffold_center_freeform_state_no_null_safety.tmpl

@@ -0,0 +1,41 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: Scaffold(
+        appBar: AppBar(title: const Text(_title)),
+        body: Center(
+          child: MyStatefulWidget(),
+        ),
+      ),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+{{code}}

dev/snippets/config/templates/stateful_widget_scaffold_center_no_null_safety.tmpl

@@ -0,0 +1,43 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: Scaffold(
+        appBar: AppBar(title: const Text(_title)),
+        body: Center(
+          child: MyStatefulWidget(),
+        ),
+      ),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> {
+  {{code}}
+}

dev/snippets/config/templates/stateful_widget_scaffold_no_null_safety.tmpl

@@ -0,0 +1,41 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: Scaffold(
+        appBar: AppBar(title: const Text(_title)),
+        body: MyStatefulWidget(),
+      ),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> {
+  {{code}}
+}

dev/snippets/config/templates/stateful_widget_ticker.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -25,7 +24,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateful widget that the main application instantiates.
 class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
+  MyStatefulWidget({Key? key}) : super(key: key);
 
   @override
   _MyStatefulWidgetState createState() => _MyStatefulWidgetState();

dev/snippets/config/templates/stateful_widget_ticker_no_null_safety.tmpl

@@ -0,0 +1,38 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/widgets.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return WidgetsApp(
+      title: 'Flutter Code Sample',
+      home: MyStatefulWidget(),
+      color: const Color(0xffffffff),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+/// AnimationControllers can be created with `vsync: this` because of TickerProviderStateMixin.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> with TickerProviderStateMixin {
+  {{code}}
+}

dev/snippets/config/templates/stateless_widget.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -27,7 +26,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key key}) : super(key: key);
+  MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_cupertino.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -27,7 +26,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key key}) : super(key: key);
+  MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_cupertino_no_null_safety.tmpl

@@ -0,0 +1,34 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/cupertino.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoApp(
+      title: _title,
+      home: MyStatelessWidget(),
+    );
+  }
+}
+
+
+{{code-preamble}}
+
+/// This is the stateless widget that the main application instantiates.
+class MyStatelessWidget extends StatelessWidget {
+  MyStatelessWidget({Key key}) : super(key: key);
+
+  @override
+  {{code}}
+}

dev/snippets/config/templates/stateless_widget_cupertino_page_scaffold.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -30,7 +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);
+  MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_cupertino_page_scaffold_no_null_safety.tmpl

@@ -0,0 +1,37 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/cupertino.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoApp(
+      title: _title,
+      home: CupertinoPageScaffold(
+        navigationBar: CupertinoNavigationBar(middle: const Text(_title)),
+        body: MyStatelessWidget(),
+      ),
+    );
+  }
+}
+
+
+{{code-preamble}}
+
+/// This is the stateless widget that the main application instantiates.
+class MyStatelessWidget extends StatelessWidget {
+  MyStatelessWidget({Key key}) : super(key: key);
+
+  @override
+  {{code}}
+}

dev/snippets/config/templates/stateless_widget_material.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -27,7 +26,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key key}) : super(key: key);
+  MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_material_no_null_safety.tmpl

@@ -0,0 +1,34 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: MyStatelessWidget(),
+    );
+  }
+}
+
+
+{{code-preamble}}
+
+/// This is the stateless widget that the main application instantiates.
+class MyStatelessWidget extends StatelessWidget {
+  MyStatelessWidget({Key key}) : super(key: key);
+
+  @override
+  {{code}}
+}

dev/snippets/config/templates/stateless_widget_no_null_safety.tmpl

@@ -0,0 +1,34 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/widgets.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return WidgetsApp(
+      title: 'Flutter Code Sample',
+      builder: (BuildContext context, Widget navigator) {
+        return MyStatelessWidget();
+      },
+      color: const Color(0xffffffff),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateless widget that the main application instantiates.
+class MyStatelessWidget extends StatelessWidget {
+  MyStatelessWidget({Key key}) : super(key: key);
+
+  @override
+  {{code}}
+}

dev/snippets/config/templates/stateless_widget_scaffold.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -30,7 +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);
+  MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_scaffold_center.tmpl

@@ -1,5 +1,4 @@
 /// Flutter code sample for {{element}}
-// @dart = 2.9
 
 {{description}}
 
@@ -32,7 +31,7 @@ class MyApp extends StatelessWidget {
 
 /// This is the stateless widget that the main application instantiates.
 class MyStatelessWidget extends StatelessWidget {
-  MyStatelessWidget({Key key}) : super(key: key);
+  MyStatelessWidget({Key? key}) : super(key: key);
 
   @override
   {{code}}

dev/snippets/config/templates/stateless_widget_scaffold_center_no_null_safety.tmpl

@@ -0,0 +1,39 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: Scaffold(
+        appBar: AppBar(title: const Text(_title)),
+        body: Center(
+          child: MyStatelessWidget(),
+        ),
+      ),
+    );
+  }
+}
+
+
+{{code-preamble}}
+
+/// This is the stateless widget that the main application instantiates.
+class MyStatelessWidget extends StatelessWidget {
+  MyStatelessWidget({Key key}) : super(key: key);
+
+  @override
+  {{code}}
+}

dev/snippets/config/templates/stateless_widget_scaffold_no_null_safety.tmpl

@@ -0,0 +1,37 @@
+/// Flutter code sample for {{element}}
+// @dart = 2.9
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: Scaffold(
+        appBar: AppBar(title: const Text(_title)),
+        body: MyStatelessWidget(),
+      ),
+    );
+  }
+}
+
+
+{{code-preamble}}
+
+/// This is the stateless widget that the main application instantiates.
+class MyStatelessWidget extends StatelessWidget {
+  MyStatelessWidget({Key key}) : super(key: key);
+
+  @override
+  {{code}}
+}

packages/flutter/lib/foundation.dart

@@ -20,6 +20,7 @@ export 'package:meta/meta.dart' show
   visibleForTesting;
 
 // Examples can assume:
+// // @dart = 2.9
 // String _name;
 // bool _first;
 // bool _lights;

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

@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 import 'tween.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // AnimationController _controller;
 
 /// The status of an animation.

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

@@ -17,6 +17,7 @@ import 'listener_helpers.dart';
 export 'package:flutter/scheduler.dart' show TickerFuture, TickerCanceled;
 
 // Examples can assume:
+// // @dart = 2.9
 // AnimationController _controller, fadeAnimationController, sizeAnimationController;
 // bool dismissed;
 // void setState(VoidCallback fn) { }

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

@@ -12,6 +12,7 @@ import 'curves.dart';
 import 'listener_helpers.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // AnimationController controller;
 
 class _AlwaysCompleteAnimation extends Animation<double> {

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

@@ -319,7 +319,7 @@ class Cubic extends Curve {
 /// part of the curve, or hardly at all in another part of the curve, depending
 /// on the definition of the curve.
 ///
-/// {@tool dartpad --template=stateless_widget_material}
+/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
 /// This example shows how to use a [Curve2D] to modify the position of a widget
 /// so that it can follow an arbitrary path.
 ///

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

@@ -12,6 +12,7 @@ import 'animations.dart';
 import 'curves.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // Animation<Offset> _animation;
 // AnimationController _controller;
 

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

@@ -7,6 +7,7 @@ import 'animation.dart';
 import 'tween.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // AnimationController myAnimationController;
 
 /// Enables creating an [Animation] whose value is defined by a sequence of

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

@@ -13,6 +13,9 @@ import 'localizations.dart';
 import 'route.dart';
 import 'theme.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 /// An application that uses Cupertino design.
 ///
 /// A convenience widget that wraps a number of widgets that are commonly

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

@@ -12,6 +12,7 @@ import 'interface_level.dart';
 import 'theme.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // Widget child;
 // BuildContext context;
 

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

@@ -75,7 +75,7 @@ enum _ContextMenuLocation {
 /// child's corners and allowing its aspect ratio to expand, similar to the
 /// Photos app on iOS.
 ///
-/// {@tool dartpad --template=stateless_widget_material}
+/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
 ///
 /// This sample shows a very simple CupertinoContextMenu for an empty red
 /// 100x100 Container. Simply long press on it to open.

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

@@ -8,6 +8,9 @@ import 'package:flutter/widgets.dart';
 import 'colors.dart';
 import 'theme.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 // Content padding determined via SwiftUI's `Form` view in the iOS 14.2 SDK.
 const EdgeInsetsGeometry _kDefaultPadding =
     EdgeInsetsDirectional.fromSTEB(20.0, 6.0, 6.0, 6.0);

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

@@ -267,7 +267,7 @@ typedef RefreshCallback = Future<void> Function();
 /// sliver such as [CupertinoSliverNavigationBar] and your main scrollable
 /// content's sliver.
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 ///
 /// When the user scrolls past [refreshTriggerPullDistance],
 /// this sample shows the default iOS pull to refresh indicator for 1 second and

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

@@ -12,6 +12,9 @@ import 'icons.dart';
 import 'localizations.dart';
 import 'text_field.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 /// A [CupertinoTextField] that mimics the look and behavior of UIKit's
 /// `UISearchTextField`.
 ///

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

@@ -11,6 +11,9 @@ import 'package:flutter/widgets.dart';
 
 import 'theme.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 // Minimum padding from edges of the segmented control to edges of
 // encompassing widget.
 const EdgeInsetsGeometry _kHorizontalItemPadding = EdgeInsets.symmetric(horizontal: 16.0);

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

@@ -14,6 +14,7 @@ import 'theme.dart';
 import 'thumb_painter.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // int _cupertinoSliderValue = 1;
 // void setState(VoidCallback fn) { }
 

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

@@ -12,6 +12,9 @@ import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 // Extracted from https://developer.apple.com/design/resources/.
 
 // Minimum padding from edges of the segmented control to edges of

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

@@ -14,6 +14,7 @@ import 'colors.dart';
 import 'thumb_painter.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // bool _lights;
 // void setState(VoidCallback fn) { }
 

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

@@ -16,6 +16,9 @@ import 'theme.dart';
 
 export 'package:flutter/services.dart' show TextInputType, TextInputAction, TextCapitalization, SmartQuotesType, SmartDashesType;
 
+// Examples can assume:
+// // @dart = 2.9
+
 const TextStyle _kDefaultPlaceholderStyle = TextStyle(
   fontWeight: FontWeight.w400,
   color: CupertinoColors.placeholderText,

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

@@ -11,6 +11,9 @@ import 'form_row.dart';
 import 'text_field.dart';
 import 'theme.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 /// Creates a [CupertinoFormRow] containing a [FormField] that wraps
 /// a [CupertinoTextField].
 ///
@@ -72,7 +75,7 @@ import 'theme.dart';
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 /// This example shows how to move the focus to the next field when the user
 /// presses the SPACE key.
 ///

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

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 // Examples can assume:
+// // @dart = 2.9
 // class Cat { }
 
 /// A category with which to annotate a class, for documentation

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

@@ -11,6 +11,7 @@ import 'print.dart';
 import 'stack_frame.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // String runtimeType;
 // bool draconisAlive;
 // bool draconisAmulet;

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

@@ -12,6 +12,7 @@ import 'debug.dart';
 import 'object.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // int rows, columns;
 // String _name;
 // bool inherit;

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

@@ -43,13 +43,13 @@ import 'theme.dart';
 ///
 /// If your application does not have a [Drawer], you should provide an
 /// affordance to call [showAboutDialog] or (at least) [showLicensePage].
-/// {@tool dartpad --template=stateless_widget_material}
+///
+/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
 ///
 /// This sample shows two ways to open [AboutDialog]. The first one
 /// uses an [AboutListTile], and the second uses the [showAboutDialog] function.
 ///
 /// ```dart
-///
 ///  Widget build(BuildContext context) {
 ///    final TextStyle textStyle = Theme.of(context).textTheme.bodyText2;
 ///    final List<Widget> aboutBoxChildren = <Widget>[
@@ -110,10 +110,9 @@ import 'theme.dart';
 ///        ),
 ///      ),
 ///    );
-///}
+/// }
 /// ```
 /// {@end-tool}
-///
 class AboutListTile extends StatelessWidget {
   /// Creates a list tile for showing an about box.
   ///

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

@@ -10,6 +10,7 @@ part of material_animated_icons;
 // generic vector graphics support in Flutter.
 
 // Examples can assume:
+// // @dart = 2.9
 // AnimationController controller;
 
 /// Shows an animated icon at a given animation [progress].

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

@@ -18,6 +18,9 @@ import 'page.dart';
 import 'scaffold.dart' show ScaffoldMessenger, ScaffoldMessengerState;
 import 'theme.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 /// [MaterialApp] uses this [TextStyle] as its [DefaultTextStyle] to encourage
 /// developers to be intentional about their [DefaultTextStyle].
 ///

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

@@ -91,7 +91,7 @@ class _ToolbarContainerLayout extends SingleChildLayoutDelegate {
 /// to false. In that case a null leading widget will result in the middle/title widget
 /// stretching to start.
 ///
-/// {@tool dartpad --template=stateless_widget_material}
+/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
 ///
 /// This sample shows an [AppBar] with two simple actions. The first action
 /// opens a [SnackBar], while the second action navigates to a new page.

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

@@ -18,7 +18,7 @@ import 'theme.dart';
 /// They are persistent and non-modal, allowing the user to either ignore them or
 /// interact with them at any time.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// ```dart
 /// Widget build(BuildContext context) {

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

@@ -13,6 +13,7 @@ import 'scaffold.dart';
 import 'theme.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // Widget bottomAppBarContents;
 
 /// A container that is typically used with [Scaffold.bottomNavigationBar], and

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

@@ -63,7 +63,7 @@ enum BottomNavigationBarType {
 ///    case it's assumed that each item will have a different background color
 ///    and that background color will contrast well with white.
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 /// This example shows a [BottomNavigationBar] as it is used within a [Scaffold]
 /// widget. The [BottomNavigationBar] has three [BottomNavigationBarItem]
 /// widgets and the [currentIndex] is set to index 0. The selected item is

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

@@ -596,7 +596,7 @@ class _BottomSheetSuspendedCurve extends ParametricCurve<double> {
 /// Returns a `Future` that resolves to the value (if any) that was passed to
 /// [Navigator.pop] when the modal bottom sheet was closed.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This example demonstrates how to use `showModalBottomSheet` to display a
 /// bottom sheet that obscures the content behind it when a user taps a button.

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

@@ -20,7 +20,7 @@ import 'theme.dart';
 /// some text describing a musical, and the other with buttons for buying
 /// tickets or listening to the show.](https://flutter.github.io/assets-for-api-docs/assets/material/card.png)
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows creation of a [Card] widget that shows album information
 /// and two actions.
@@ -63,7 +63,7 @@ import 'theme.dart';
 /// Sometimes the primary action area of a card is the card itself. Cards can be
 /// one large touch target that shows a detail screen when tapped.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows creation of a [Card] widget that can be tapped. When
 /// tapped this [Card]'s [InkWell] displays an "ink splash" that fills the

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

@@ -10,6 +10,7 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // void setState(VoidCallback fn) { }
 
 /// A [ListTile] with a [Checkbox]. In other words, a checkbox with a label.
@@ -39,7 +40,7 @@ import 'theme_data.dart';
 /// To show the [CheckboxListTile] as disabled, pass null as the [onChanged]
 /// callback.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
 ///
 /// ![CheckboxListTile sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile.png)
 ///
@@ -84,7 +85,7 @@ import 'theme_data.dart';
 /// into one. Therefore, it may be necessary to create a custom radio tile
 /// widget to accommodate similar use cases.
 ///
-/// {@tool sample --template=stateful_widget_scaffold_center}
+/// {@tool sample --template=stateful_widget_scaffold_center_no_null_safety}
 ///
 /// ![Checkbox list tile semantics sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile_semantics.png)
 ///
@@ -168,7 +169,7 @@ import 'theme_data.dart';
 /// combining [Checkbox] with other widgets, such as [Text], [Padding] and
 /// [InkWell].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
 ///
 /// ![Custom checkbox list tile sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile_custom.png)
 ///

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

@@ -23,6 +23,9 @@ import 'theme.dart';
 import 'theme_data.dart';
 import 'tooltip.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 // Some design constants
 const double _kChipHeight = 32.0;
 const double _kDeleteIconSize = 18.0;
@@ -221,7 +224,7 @@ abstract class DeletableChipAttributes {
   /// that the user tapped the delete button. In order to delete the chip, you
   /// have to do something similar to the following sample:
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold_center}
+  /// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
   ///
   /// This sample shows how to use [onDeleted] to remove an entry when the
   /// delete button is tapped.

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

@@ -9,6 +9,7 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // String userAvatarUrl;
 
 /// A circle that represents a user.

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

@@ -6,6 +6,9 @@ import 'dart:ui' show Color;
 
 import 'package:flutter/painting.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 /// Defines a single color as well a color swatch with ten shades of the color.
 ///
 /// The color's shades are referred to by index. The greater the index, the

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

@@ -260,7 +260,7 @@ class DataCell {
 /// [PaginatedDataTable] which automatically splits the data into
 /// multiple pages.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows how to display a [DataTable] with three columns: name, age, and
 /// role. The columns are defined by three [DataColumn] objects. The table
@@ -322,7 +322,7 @@ class DataCell {
 /// {@end-tool}
 ///
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold}
+/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
 ///
 /// This sample shows how to display a [DataTable] with alternate colors per
 /// row, and a custom color for when the row is selected.

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

@@ -23,6 +23,7 @@ import 'theme.dart';
 
 
 // Examples can assume:
+// // @dart = 2.9
 // BuildContext context;
 
 const Duration _kMonthScrollDuration = Duration(milliseconds: 200);

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

@@ -18,6 +18,7 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // enum Department { treasury, state }
 // BuildContext context;
 

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

@@ -9,6 +9,7 @@ import 'divider_theme.dart';
 import 'theme.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // BuildContext context;
 
 /// A thin horizontal line, with padding on either side.
@@ -24,7 +25,7 @@ import 'theme.dart';
 /// The box's total height is controlled by [height]. The appropriate
 /// padding is automatically computed from the height.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows how to display a Divider between an orange and blue box
 /// inside a column. The Divider is 20 logical pixels in height and contains a

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

@@ -728,7 +728,7 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 /// dropdown's value. It should also call [State.setState] to rebuild the
 /// dropdown with the new value.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
 ///
 /// This sample shows a `DropdownButton` with a large arrow icon,
 /// purple text style, and bold purple underline, whose value is one of "One",
@@ -912,7 +912,7 @@ class DropdownButton<T> extends StatefulWidget {
   /// from the list corresponds to the [DropdownMenuItem] of the same index
   /// in [items].
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold}
+  /// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
   ///
   /// This sample shows a `DropdownButton` with a button with [Text] that
   /// corresponds to but is unique from [DropdownMenuItem].
@@ -963,7 +963,7 @@ class DropdownButton<T> extends StatefulWidget {
   /// To use a separate text style for selected item when it's displayed within
   /// the dropdown button, consider using [selectedItemBuilder].
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold}
+  /// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
   ///
   /// This sample shows a `DropdownButton` with a dropdown button text style
   /// that is different than its menu items.

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

@@ -148,7 +148,7 @@ class ExpansionPanelRadio extends ExpansionPanel {
 /// Note that [expansionCallback] behaves differently for [ExpansionPanelList]
 /// and [ExpansionPanelList.radio].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold}
+/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
 ///
 /// Here is a simple example of how to implement ExpansionPanelList.
 ///
@@ -252,7 +252,7 @@ class ExpansionPanelList extends StatefulWidget {
   /// arguments must not be null. The [children] objects must be instances
   /// of [ExpansionPanelRadio].
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold}
+  /// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
   ///
   /// Here is a simple example of how to implement ExpansionPanelList.radio.
   ///

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

@@ -53,7 +53,7 @@ enum StretchMode {
 /// [FlexibleSpaceBar.createSettings], to convey sizing information down to the
 /// [FlexibleSpaceBar].
 ///
-/// {@tool dartpad --template=freeform}
+/// {@tool dartpad --template=freeform_no_null_safety}
 /// This sample application demonstrates the different features of the
 /// [FlexibleSpaceBar] when used in a [SliverAppBar]. This app bar is configured
 /// to stretch into the overscroll space, and uses the

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

@@ -58,7 +58,7 @@ class _DefaultHeroTag {
 /// disabled. Consider changing the [backgroundColor] if disabling the floating
 /// action button.
 ///
-/// {@tool dartpad --template=stateless_widget_material}
+/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
 /// This example shows how to display a [FloatingActionButton] in a
 /// [Scaffold], with a pink [backgroundColor] and a thumbs up [Icon].
 ///
@@ -85,7 +85,7 @@ class _DefaultHeroTag {
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_material}
+/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
 /// This example shows how to make an extended [FloatingActionButton] in a
 /// [Scaffold], with a  pink [backgroundColor], a thumbs up [Icon] and a
 /// [Text] label that reads "Approve".

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

@@ -431,7 +431,7 @@ abstract class FloatingActionButtonLocation {
 /// You can create your own subclass of [StandardFabLocation]
 /// to implement a custom [FloatingActionButtonLocation].
 ///
-/// {@tool dartpad --template=stateless_widget_material}
+/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
 ///
 /// This is an example of a user-defined [FloatingActionButtonLocation].
 ///

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

@@ -40,7 +40,7 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 /// requirements in the Material Design specification. The [alignment] controls
 /// how the icon itself is positioned within the hit region.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
 ///
 /// This sample shows an `IconButton` that uses the Material icon "volume_up" to
 /// increase the volume.
@@ -84,7 +84,7 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 /// the underlying [Material] along with the splash and highlight
 /// [InkResponse] contributed by descendant widgets.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// In this sample the icon button's background color is defined with an [Ink]
 /// widget whose child is an [IconButton]. The icon button's filled background

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

@@ -1170,7 +1170,7 @@ class _InkResponseState extends State<_InkResponseStateWidget>
 ///
 /// An example of this situation is as follows:
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
 ///
 /// Tap the container to cause it to grow. Then, tap it again and hold before
 /// the widget reaches its maximum size to observe the clipped ink splash.

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

@@ -2389,7 +2389,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
 /// to describe their decoration. (In fact, this class is merely the
 /// configuration of an [InputDecorator], which does all the heavy lifting.)
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows how to style a `TextField` using an `InputDecorator`. The
 /// TextField displays a "send message" icon to the left of the input area,
@@ -2414,7 +2414,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows how to style a "collapsed" `TextField` using an
 /// `InputDecorator`. The collapsed `TextField` surrounds the hint text and
@@ -2434,7 +2434,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows how to create a `TextField` with hint text, a red border
 /// on all sides, and an error message. To display a red border and error
@@ -2455,7 +2455,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows how to style a `TextField` with a round border and
 /// additional text before and after the input area. It displays "Prefix" before
@@ -2836,7 +2836,7 @@ class InputDecoration {
   /// setting the constraints' minimum height and width to a value lower than
   /// 48px.
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold}
+  /// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
   /// This example shows the differences between two `TextField` widgets when
   /// [prefixIconConstraints] is set to the default value and when one is not.
   ///
@@ -3004,7 +3004,7 @@ class InputDecoration {
   /// If null, a [BoxConstraints] with a minimum width and height of 48px is
   /// used.
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold}
+  /// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
   /// This example shows the differences between two `TextField` widgets when
   /// [suffixIconConstraints] is set to the default value and when one is not.
   ///

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

@@ -416,7 +416,7 @@ enum ListTileControlAffinity {
 /// you're looking for, it's easy to create custom list items with a
 /// combination of other widgets, such as [Row]s and [Column]s.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// Here is an example of a custom list item that resembles a YouTube-related
 /// video list item created with [Expanded] and [Container] widgets.
@@ -537,7 +537,7 @@ enum ListTileControlAffinity {
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// Here is an example of an article list item with multiline titles and
 /// subtitles. It utilizes [Row]s and [Column]s, as well as [Expanded] and
@@ -889,7 +889,7 @@ class ListTile extends StatelessWidget {
   /// By default the selected color is the theme's primary color. The selected color
   /// can be overridden with a [ListTileTheme].
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold}
+  /// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
   ///
   /// Here is an example of using a [StatefulWidget] to keep track of the
   /// selected index, and using that to set the `selected` property on the

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

@@ -179,7 +179,7 @@ class _MaterialStateColor extends MaterialStateColor {
 /// To use a [MaterialStateMouseCursor], you should create a subclass of
 /// [MaterialStateMouseCursor] and implement the abstract `resolve` method.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center}
+/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
 ///
 /// This example defines a mouse cursor that resolves to
 /// [SystemMouseCursors.forbidden] when its widget is disabled.
@@ -294,7 +294,7 @@ class _EnabledAndDisabledMouseCursor extends MaterialStateMouseCursor {
 /// To use a [MaterialStateBorderSide], you should create a subclass of a
 /// [MaterialStateBorderSide] and override the abstract `resolve` method.
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 ///
 /// This example defines a subclass of [MaterialStateBorderSide], that resolves
 /// to a red border side when its widget is selected.
@@ -352,7 +352,7 @@ abstract class MaterialStateBorderSide extends BorderSide implements MaterialSta
 /// [OutlinedBorder] and implement [MaterialStateOutlinedBorder]'s abstract
 /// `resolve` method.
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 ///
 /// This example defines a subclass of [RoundedRectangleBorder] and an
 /// implementation of [MaterialStateOutlinedBorder], that resolves to
@@ -423,7 +423,7 @@ abstract class MaterialStateOutlinedBorder extends OutlinedBorder implements Mat
 /// of their current material state and [resolve] the button style's
 /// material state properties when their value is needed.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center}
+/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
 ///
 /// This example shows how you can override the default text and icon
 /// color (the "foreground color") of a [TextButton] with a

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

@@ -16,6 +16,9 @@ import 'navigation_rail_theme.dart';
 import 'theme.dart';
 import 'theme_data.dart';
 
+// Examples can assume:
+// // @dart = 2.9
+
 /// A material widget that is meant to be displayed at the left or right of an
 /// app to navigate between a small number of views, typically between three and
 /// five.
@@ -38,7 +41,7 @@ import 'theme_data.dart';
 /// [https://github.com/flutter/samples/blob/master/experimental/web_dashboard/lib/src/widgets/third_party/adaptive_scaffold.dart]
 /// for an example.
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 ///
 /// This example shows a [NavigationRail] used within a Scaffold with 3
 /// [NavigationRailDestination]s. The main content is separated by a divider

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

@@ -57,7 +57,7 @@ const Duration _kElevationDuration = Duration(milliseconds: 75);
 /// Outline buttons have a minimum size of 88.0 by 36.0 which can be overridden
 /// with [ButtonTheme].
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center}
+/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
 ///
 /// Here is an example of a basic [OutlineButton].
 ///

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

@@ -21,6 +21,7 @@ import 'theme.dart';
 import 'tooltip.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // enum Commands { heroAndScholar, hurricaneCame }
 // dynamic _heroAndScholar;
 // dynamic _selection;

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

@@ -243,7 +243,7 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
 /// The minimum height of the indicator can be specified using [minHeight].
 /// The indicator can be made taller by wrapping the widget with a [SizedBox].
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker}
+/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
 ///
 /// This example shows a [LinearProgressIndicator] with a changing value.
 ///
@@ -481,7 +481,7 @@ class _CircularProgressIndicatorPainter extends CustomPainter {
 /// The indicator arc is displayed with [valueColor], an animated value. To
 /// specify a constant color use: `AlwaysStoppedAnimation<Color>(color)`.
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker}
+/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
 ///
 /// This example shows a [CircularProgressIndicator] with a changing value.
 ///

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

@@ -30,7 +30,7 @@ const double _kInnerRadius = 4.5;
 /// will respond to [onChanged] by calling [State.setState] to update the
 /// radio button's [groupValue].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
 ///
 /// Here is an example of Radio widgets wrapped in ListTiles, which is similar
 /// to what you could get with the RadioListTile widget.
@@ -202,7 +202,7 @@ class Radio<T> extends StatefulWidget {
   ///
   /// The default is false.
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold}
+  /// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
   /// This example shows how to enable deselecting a radio button by setting the
   /// [toggleable] attribute.
   ///

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

@@ -10,6 +10,7 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // void setState(VoidCallback fn) { }
 
 /// A [ListTile] with a [Radio]. In other words, a radio button with a label.
@@ -40,7 +41,7 @@ import 'theme_data.dart';
 /// To show the [RadioListTile] as disabled, pass null as the [onChanged]
 /// callback.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold}
+/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
 ///
 /// ![RadioListTile sample](https://flutter.github.io/assets-for-api-docs/assets/material/radio_list_tile.png)
 ///
@@ -92,7 +93,7 @@ import 'theme_data.dart';
 /// into one. Therefore, it may be necessary to create a custom radio tile
 /// widget to accommodate similar use cases.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold}
+/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
 ///
 /// ![Radio list tile semantics sample](https://flutter.github.io/assets-for-api-docs/assets/material/radio_list_tile_semantics.png)
 ///
@@ -196,7 +197,7 @@ import 'theme_data.dart';
 /// combining [Radio] with other widgets, such as [Text], [Padding] and
 /// [InkWell].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold}
+/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
 ///
 /// ![Custom radio list tile sample](https://flutter.github.io/assets-for-api-docs/assets/material/radio_list_tile_custom.png)
 ///
@@ -386,7 +387,7 @@ class RadioListTile<T> extends StatelessWidget {
   ///
   /// The default is false.
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold}
+  /// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
   /// This example shows how to enable deselecting a radio button by setting the
   /// [toggleable] attribute.
   ///

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

@@ -43,7 +43,7 @@ import 'theme_data.dart';
 /// Raised buttons have a minimum size of 88.0 by 36.0 which can be overridden
 /// with [ButtonTheme].
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold}
+/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
 ///
 /// This sample shows how to render a disabled RaisedButton, an enabled RaisedButton
 /// and lastly a RaisedButton with gradient background.

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

@@ -18,6 +18,7 @@ import 'slider_theme.dart';
 import 'theme.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // RangeValues _rangeValues = RangeValues(0.3, 0.7);
 // RangeValues _dollarsRange = RangeValues(50, 100);
 // void setState(VoidCallback fn) { }
@@ -33,7 +34,7 @@ typedef PaintRangeValueIndicator = void Function(PaintingContext context, Offset
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=ufb4gIPDmEs}
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold}
+/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
 ///
 /// ![A range slider widget, consisting of 5 divisions and showing the default
 /// value indicator.](https://flutter.github.io/assets-for-api-docs/assets/material/range_slider.png)

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

@@ -12,6 +12,7 @@ import 'material.dart';
 import 'material_localizations.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // class MyDataObject { }
 
 /// The callback used by [ReorderableListView] to move an item to a new
@@ -59,7 +60,7 @@ typedef ReorderCallback = void Function(int oldIndex, int newIndex);
 /// The [onReorder] parameter is required and will be called when a child
 /// widget is dragged to a new position.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold}
+/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
 ///
 /// ```dart
 /// List<String> _list = List.generate(5, (i) => "${i}");

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

@@ -30,6 +30,7 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 // Examples can assume:
+// // @dart = 2.9
 // TabController tabController;
 // void setState(VoidCallback fn) { }
 // String appBarTitle;
@@ -68,7 +69,7 @@ enum _ScaffoldSlot {
 /// [BuildContext] via [ScaffoldMessenger.of] and use the
 /// [ScaffoldMessengerState.showSnackBar] function.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center}
+/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
 ///
 /// Here is an example of showing a [SnackBar] when the user presses a button.
 ///
@@ -111,7 +112,7 @@ class ScaffoldMessenger extends StatefulWidget {
   /// The state from the closest instance of this class that encloses the given
   /// context.
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold_center}
+  /// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
   /// Typical usage of the [ScaffoldMessenger.of] function is to call it in
   /// response to a user gesture or an application state change.
   ///
@@ -137,7 +138,7 @@ class ScaffoldMessenger extends StatefulWidget {
   /// function. The [MaterialApp.scaffoldMessengerKey] refers to the root
   /// ScaffoldMessenger that is provided by default.
   ///
-  /// {@tool dartpad --template=freeform}
+  /// {@tool dartpad --template=freeform_no_null_safety}
   /// Sometimes [SnackBar]s are produced by code that doesn't have ready access
   /// to a valid [BuildContext]. One such example of this is when you show a
   /// SnackBar from a method outside of the `build` function. In these
@@ -303,7 +304,7 @@ class ScaffoldMessengerState extends State<ScaffoldMessenger> with TickerProvide
   /// See [ScaffoldMessenger.of] for information about how to obtain the
   /// [ScaffoldMessengerState].
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold_center}
+  /// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
   ///
   /// Here is an example of showing a [SnackBar] when the user presses a button.
   ///
@@ -1258,7 +1259,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// [ScaffoldState] for the current [BuildContext] via [Scaffold.of] and use the
 /// [ScaffoldState.showBottomSheet] function.
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 /// This example shows a [Scaffold] with a [body] and [FloatingActionButton].
 /// The [body] is a [Text] placed in a [Center] in order to center the text
 /// within the [Scaffold]. The [FloatingActionButton] is connected to a
@@ -1287,7 +1288,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 /// This example shows a [Scaffold] with a blueGrey [backgroundColor], [body]
 /// and [FloatingActionButton]. The [body] is a [Text] placed in a [Center] in
 /// order to center the text within the [Scaffold]. The [FloatingActionButton]
@@ -1317,7 +1318,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material}
+/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
 /// This example shows a [Scaffold] with an [AppBar], a [BottomAppBar] and a
 /// [FloatingActionButton]. The [body] is a [Text] placed in a [Center] in order
 /// to center the text within the [Scaffold]. The [FloatingActionButton] is
@@ -1559,7 +1560,7 @@ class Scaffold extends StatefulWidget {
   ///
   /// To close the drawer, use [Navigator.pop].
   ///
-  /// {@tool dartpad --template=stateful_widget_material}
+  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
   /// To disable the drawer edge swipe, set the
   /// [Scaffold.drawerEnableOpenDragGesture] to false. Then, use
   /// [ScaffoldState.openDrawer] to open the drawer and [Navigator.pop] to close
@@ -1622,7 +1623,7 @@ class Scaffold extends StatefulWidget {
   ///
   /// To close the drawer, use [Navigator.pop].
   ///
-  /// {@tool dartpad --template=stateful_widget_material}
+  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
   /// To disable the drawer edge swipe, set the
   /// [Scaffold.endDrawerEnableOpenDragGesture] to false. Then, use
   /// [ScaffoldState.openEndDrawer] to open the drawer and [Navigator.pop] to
@@ -1789,7 +1790,7 @@ class Scaffold extends StatefulWidget {
   /// If no instance of this class encloses the given context, will cause an
   /// assert in debug mode, and throw an exception in release mode.
   ///
-  /// {@tool dartpad --template=freeform}
+  /// {@tool dartpad --template=freeform_no_null_safety}
   /// Typical usage of the [Scaffold.of] function is to call it from within the
   /// `build` method of a child of a [Scaffold].
   ///
@@ -1860,7 +1861,7 @@ class Scaffold extends StatefulWidget {
   /// ```
   /// {@end-tool}
   ///
-  /// {@tool dartpad --template=stateless_widget_material}
+  /// {@tool dartpad --template=stateless_widget_material_no_null_safety}
   /// When the [Scaffold] is actually created in the same `build` function, the
   /// `context` argument to the `build` function can't be used to find the
   /// [Scaffold] (since it's "above" the widget being returned in the widget
@@ -2172,7 +2173,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   /// See [ScaffoldMessenger.of] for information about how to obtain the
   /// [ScaffoldMessengerState].
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold_center}
+  /// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
   ///
   /// Here is an example of showing a [SnackBar] when the user presses a button.
   ///
@@ -2546,7 +2547,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   /// of the app. Modal bottom sheets can be created and displayed with the
   /// [showModalBottomSheet] function.
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold}
+  /// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
   ///
   /// This example demonstrates how to use `showBottomSheet` to display a
   /// bottom sheet when a user taps a button. It also demonstrates how to