diff --git a/packages/flutter/test/widgets/editable_text_test.dart b/packages/flutter/test/widgets/editable_text_test.dart
index 9276eede19..7defd6cd7e 100644
--- a/packages/flutter/test/widgets/editable_text_test.dart
+++ b/packages/flutter/test/widgets/editable_text_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/gestures.dart';
@@ -24,7 +22,7 @@ class _MatchesMethodCall extends Matcher {
   const _MatchesMethodCall(this.name, {this.arguments});
 
   final String name;
-  final Matcher arguments;
+  final Matcher? arguments;
 
   @override
   bool matches(dynamic item, Map<dynamic, dynamic> matchState) {
@@ -42,7 +40,7 @@ class _MatchesMethodCall extends Matcher {
   }
 }
 
-TextEditingController controller;
+late TextEditingController controller;
 final FocusNode focusNode = FocusNode(debugLabel: 'EditableText Node');
 final FocusScopeNode focusScopeNode = FocusScopeNode(debugLabel: 'EditableText Scope Node');
 const TextStyle textStyle = TextStyle();
@@ -62,7 +60,7 @@ class MockClipboard {
       case 'Clipboard.getData':
         return _clipboardData;
       case 'Clipboard.setData':
-        _clipboardData = methodCall.arguments;
+        _clipboardData = methodCall.arguments as Object;
         break;
     }
   }
@@ -83,7 +81,6 @@ void main() {
 
   tearDown(() {
     controller.dispose();
-    controller = null;
   });
 
   // Tests that the desired keyboard action button is requested.
@@ -92,9 +89,9 @@ void main() {
   // requests [serializedActionName] when attaching to the platform's input
   // system.
   Future<void> _desiredKeyboardActionIsRequested({
-    WidgetTester tester,
-    TextInputAction action,
-    String serializedActionName,
+    required WidgetTester tester,
+    TextInputAction? action,
+    String serializedActionName = '',
   }) async {
     await tester.pumpWidget(
       MediaQuery(
@@ -121,8 +118,8 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     controller.text = 'test';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals(serializedActionName));
   }
 
@@ -182,10 +179,10 @@ void main() {
     final EditableText editableText =
         tester.firstWidget(find.byType(EditableText));
     expect(editableText.maxLines, equals(1));
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputType']['name'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputType']['name'],
         equals('TextInputType.text'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals('TextInputAction.done'));
   });
 
@@ -321,9 +318,9 @@ void main() {
         await tester.showKeyboard(find.byType(EditableText));
         controller.text = 'test';
         await tester.idle();
-        expect(tester.testTextInput.editingState['text'], equals('test'));
+        expect(tester.testTextInput.editingState!['text'], equals('test'));
         expect(
-          tester.testTextInput.setClientArgs['inputType']['name'],
+          tester.testTextInput.setClientArgs!['inputType']['name'],
           // On web, we don't infer the keyboard type as "name". We only infer
           // on iOS and macOS.
           kIsWeb ? equals('TextInputType.address') : equals('TextInputType.name'),
@@ -357,8 +354,8 @@ void main() {
         await tester.showKeyboard(find.byType(EditableText));
         controller.text = 'test';
         await tester.idle();
-        expect(tester.testTextInput.editingState['text'], equals('test'));
-        expect(tester.testTextInput.setClientArgs['inputType']['name'], equals('TextInputType.address'));
+        expect(tester.testTextInput.editingState!['text'], equals('test'));
+        expect(tester.testTextInput.setClientArgs!['inputType']['name'], equals('TextInputType.address'));
       });
 
     testWidgets('inferred keyboard types can be overridden: ios',
@@ -389,8 +386,8 @@ void main() {
         await tester.showKeyboard(find.byType(EditableText));
         controller.text = 'test';
         await tester.idle();
-        expect(tester.testTextInput.editingState['text'], equals('test'));
-        expect(tester.testTextInput.setClientArgs['inputType']['name'], equals('TextInputType.text'));
+        expect(tester.testTextInput.editingState!['text'], equals('test'));
+        expect(tester.testTextInput.setClientArgs!['inputType']['name'], equals('TextInputType.text'));
     }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
 
     testWidgets('inferred keyboard types can be overridden: non-ios',
@@ -421,8 +418,8 @@ void main() {
         await tester.showKeyboard(find.byType(EditableText));
         controller.text = 'test';
         await tester.idle();
-        expect(tester.testTextInput.editingState['text'], equals('test'));
-        expect(tester.testTextInput.setClientArgs['inputType']['name'], equals('TextInputType.text'));
+        expect(tester.testTextInput.editingState!['text'], equals('test'));
+        expect(tester.testTextInput.setClientArgs!['inputType']['name'], equals('TextInputType.text'));
     });
   });
 
@@ -452,10 +449,10 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     controller.text = 'test';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputType']['name'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputType']['name'],
         equals('TextInputType.multiline'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals('TextInputAction.newline'));
   });
 
@@ -485,10 +482,10 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     controller.text = 'test';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputType']['name'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputType']['name'],
         equals('TextInputType.visiblePassword'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals('TextInputAction.done'));
   });
 
@@ -519,7 +516,7 @@ void main() {
     await tester.tap(find.byType(EditableText));
     await tester.showKeyboard(find.byType(EditableText));
     await tester.idle();
-    expect(tester.testTextInput.setClientArgs['enableSuggestions'], enableSuggestions);
+    expect(tester.testTextInput.setClientArgs!['enableSuggestions'], enableSuggestions);
   });
 
   group('smartDashesType and smartQuotesType', () {
@@ -552,8 +549,8 @@ void main() {
       await tester.tap(find.byType(EditableText));
       await tester.showKeyboard(find.byType(EditableText));
       await tester.idle();
-      expect(tester.testTextInput.setClientArgs['smartDashesType'], smartDashesType.index.toString());
-      expect(tester.testTextInput.setClientArgs['smartQuotesType'], smartQuotesType.index.toString());
+      expect(tester.testTextInput.setClientArgs!['smartDashesType'], smartDashesType.index.toString());
+      expect(tester.testTextInput.setClientArgs!['smartQuotesType'], smartQuotesType.index.toString());
     });
 
     testWidgets('default to true when obscureText is false', (WidgetTester tester) async {
@@ -582,8 +579,8 @@ void main() {
       await tester.tap(find.byType(EditableText));
       await tester.showKeyboard(find.byType(EditableText));
       await tester.idle();
-      expect(tester.testTextInput.setClientArgs['smartDashesType'], '1');
-      expect(tester.testTextInput.setClientArgs['smartQuotesType'], '1');
+      expect(tester.testTextInput.setClientArgs!['smartDashesType'], '1');
+      expect(tester.testTextInput.setClientArgs!['smartQuotesType'], '1');
     });
 
     testWidgets('default to false when obscureText is true', (WidgetTester tester) async {
@@ -612,8 +609,8 @@ void main() {
       await tester.tap(find.byType(EditableText));
       await tester.showKeyboard(find.byType(EditableText));
       await tester.idle();
-      expect(tester.testTextInput.setClientArgs['smartDashesType'], '0');
-      expect(tester.testTextInput.setClientArgs['smartQuotesType'], '0');
+      expect(tester.testTextInput.setClientArgs!['smartDashesType'], '0');
+      expect(tester.testTextInput.setClientArgs!['smartQuotesType'], '0');
     });
   });
 
@@ -786,10 +783,10 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     controller.text = 'test';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputType']['name'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputType']['name'],
         equals('TextInputType.multiline'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals('TextInputAction.newline'));
   });
 
@@ -820,10 +817,10 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     controller.text = 'test';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputType']['name'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputType']['name'],
         equals('TextInputType.text'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals('TextInputAction.done'));
   });
 
@@ -854,10 +851,10 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     controller.text = 'test';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputType']['name'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputType']['name'],
         equals('TextInputType.phone'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals('TextInputAction.done'));
   });
 
@@ -887,10 +884,10 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     controller.text = 'test';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputType']['name'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputType']['name'],
         equals('TextInputType.multiline'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals('TextInputAction.newline'));
   });
 
@@ -920,10 +917,10 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     controller.text = 'test';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('test'));
-    expect(tester.testTextInput.setClientArgs['inputType']['name'],
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
+    expect(tester.testTextInput.setClientArgs!['inputType']['name'],
         equals('TextInputType.text'));
-    expect(tester.testTextInput.setClientArgs['inputAction'],
+    expect(tester.testTextInput.setClientArgs!['inputAction'],
         equals('TextInputAction.done'));
   });
 
@@ -956,7 +953,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals('test'));
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
     expect(state.wantKeepAlive, true);
 
     tester.testTextInput.log.clear();
@@ -1000,7 +997,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals('test3'));
+    expect(tester.testTextInput.editingState!['text'], equals('test3'));
     expect(state.wantKeepAlive, true);
 
     tester.testTextInput.log.clear();
@@ -1018,7 +1015,7 @@ void main() {
     await tester.showKeyboard(find.byType(EditableText));
     await tester.pump();
     controller.text = 'test2';
-    expect(tester.testTextInput.editingState['text'], equals('test2'));
+    expect(tester.testTextInput.editingState!['text'], equals('test2'));
     // Widget regained the focus.
     expect(state.wantKeepAlive, true);
   });
@@ -1071,7 +1068,7 @@ void main() {
     controller.text = 'test';
     await tester.idle();
 
-    expect(tester.testTextInput.editingState['text'], equals('test'));
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
     final EditableTextState state =
         tester.state<EditableTextState>(find.byWidget(testNameField));
     expect(state.wantKeepAlive, true);
@@ -1093,7 +1090,7 @@ void main() {
     await tester.showKeyboard(find.byWidget(testPhoneField));
     controller.text = '650123123';
     await tester.idle();
-    expect(tester.testTextInput.editingState['text'], equals('650123123'));
+    expect(tester.testTextInput.editingState!['text'], equals('650123123'));
     // Widget regained the focus.
     expect(state.wantKeepAlive, true);
   });
@@ -1364,13 +1361,13 @@ void main() {
       // On the web, a regular connection to the platform should've been made
       // with the `readOnly` flag set to true.
       expect(tester.testTextInput.hasAnyClients, isTrue);
-      expect(tester.testTextInput.setClientArgs['readOnly'], isTrue);
+      expect(tester.testTextInput.setClientArgs!['readOnly'], isTrue);
       expect(
-        tester.testTextInput.editingState['text'],
+        tester.testTextInput.editingState!['text'],
         'Lorem ipsum dolor sit amet',
       );
-      expect(tester.testTextInput.editingState['selectionBase'], 0);
-      expect(tester.testTextInput.editingState['selectionExtent'], 5);
+      expect(tester.testTextInput.editingState!['selectionBase'], 0);
+      expect(tester.testTextInput.editingState!['selectionExtent'], 5);
     } else {
       // On non-web platforms, a read-only field doesn't need a connection with
       // the platform.
@@ -1421,7 +1418,7 @@ void main() {
   });
 
   testWidgets('Read-only fields do not format text', (WidgetTester tester) async {
-    SelectionChangedCause selectionCause;
+    late SelectionChangedCause selectionCause;
 
     final TextEditingController controller =
         TextEditingController(text: 'Lorem ipsum dolor sit amet');
@@ -1436,8 +1433,8 @@ void main() {
           style: textStyle,
           cursorColor: cursorColor,
           selectionControls: materialTextSelectionControls,
-          onSelectionChanged: (TextSelection selection, SelectionChangedCause cause) {
-            selectionCause = cause;
+          onSelectionChanged: (TextSelection selection, SelectionChangedCause? cause) {
+            selectionCause = cause!;
           },
         ),
       ),
@@ -1462,7 +1459,7 @@ void main() {
 
   testWidgets('Sends "updateConfig" when read-only flag is flipped', (WidgetTester tester) async {
     bool readOnly = true;
-    StateSetter setState;
+    late StateSetter setState;
     final TextEditingController controller = TextEditingController(text: 'Lorem ipsum dolor sit amet');
 
     await tester.pumpWidget(
@@ -1488,24 +1485,24 @@ void main() {
 
     expect(tester.testTextInput.hasAnyClients, kIsWeb ? isTrue : isFalse);
     if (kIsWeb) {
-      expect(tester.testTextInput.setClientArgs['readOnly'], isTrue);
+      expect(tester.testTextInput.setClientArgs!['readOnly'], isTrue);
     }
 
     setState(() { readOnly = false; });
     await tester.pump();
 
     expect(tester.testTextInput.hasAnyClients, isTrue);
-    expect(tester.testTextInput.setClientArgs['readOnly'], isFalse);
+    expect(tester.testTextInput.setClientArgs!['readOnly'], isFalse);
   });
 
   testWidgets('Fires onChanged when text changes via TextSelectionOverlay', (WidgetTester tester) async {
-    String changedValue;
+    late String changedValue;
     final Widget widget = MaterialApp(
       home: EditableText(
         backgroundCursorColor: Colors.grey,
         controller: TextEditingController(),
         focusNode: FocusNode(),
-        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
         cursorColor: Colors.blue,
         selectionControls: materialTextSelectionControls,
         keyboardType: TextInputType.text,
@@ -1559,12 +1556,12 @@ void main() {
       TextInputAction.previous: true,
     };
 
-    final TextInputAction action = focusVariants.currentValue;
+    final TextInputAction action = focusVariants.currentValue!;
     expect(actionShouldLoseFocus.containsKey(action), isTrue);
 
     Future<void> _ensureCorrectFocusHandlingForAction(
         TextInputAction action, {
-          @required bool shouldLoseFocus,
+          required bool shouldLoseFocus,
           bool shouldFocusNext = false,
           bool shouldFocusPrevious = false,
         }) async {
@@ -1582,7 +1579,7 @@ void main() {
               backgroundCursorColor: Colors.grey,
               controller: TextEditingController(),
               focusNode: focusNode,
-              style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+              style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
               cursorColor: Colors.blue,
               selectionControls: materialTextSelectionControls,
               keyboardType: TextInputType.text,
@@ -1600,15 +1597,15 @@ void main() {
       await tester.testTextInput.receiveAction(action);
       await tester.pump();
 
-      expect(Focus.of(nextKey.currentContext).hasFocus, equals(shouldFocusNext));
-      expect(Focus.of(previousKey.currentContext).hasFocus, equals(shouldFocusPrevious));
+      expect(Focus.of(nextKey.currentContext!)!.hasFocus, equals(shouldFocusNext));
+      expect(Focus.of(previousKey.currentContext!)!.hasFocus, equals(shouldFocusPrevious));
       expect(focusNode.hasFocus, equals(!shouldLoseFocus));
     }
 
     try {
       await _ensureCorrectFocusHandlingForAction(
         action,
-        shouldLoseFocus: actionShouldLoseFocus[action],
+        shouldLoseFocus: actionShouldLoseFocus[action]!,
         shouldFocusNext: action == TextInputAction.next,
         shouldFocusPrevious: action == TextInputAction.previous,
       );
@@ -1626,7 +1623,7 @@ void main() {
         backgroundCursorColor: Colors.grey,
         controller: TextEditingController(),
         focusNode: focusNode,
-        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
         cursorColor: Colors.blue,
         selectionControls: materialTextSelectionControls,
         keyboardType: TextInputType.text,
@@ -1663,7 +1660,7 @@ void main() {
         backgroundCursorColor: Colors.grey,
         controller: TextEditingController(),
         focusNode: focusNode,
-        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
         cursorColor: Colors.blue,
         onEditingComplete: () {
           onEditingCompleteCalled = true;
@@ -1703,7 +1700,7 @@ void main() {
         backgroundCursorColor: Colors.grey,
         controller: TextEditingController(),
         focusNode: focusNode,
-        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
         cursorColor: Colors.blue,
         onEditingComplete: () {
           onEditingCompleteCalled = true;
@@ -1743,7 +1740,7 @@ void main() {
         backgroundCursorColor: Colors.grey,
         controller: TextEditingController(),
         focusNode: focusNode,
-        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
         cursorColor: Colors.blue,
         maxLines: 1,
         onEditingComplete: () {
@@ -1783,7 +1780,7 @@ void main() {
         backgroundCursorColor: Colors.grey,
         controller: TextEditingController(),
         focusNode: focusNode,
-        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
         cursorColor: Colors.blue,
         maxLines: 3,
         onEditingComplete: () {
@@ -1818,7 +1815,7 @@ void main() {
     (WidgetTester tester) async {
       const Color rectColor = Color(0xFFFF0000);
 
-      void verifyAutocorrectionRectVisibility({ bool expectVisible }) {
+      void verifyAutocorrectionRectVisibility({ required bool expectVisible }) {
         PaintPattern evaluate() {
           if (expectVisible) {
             return paints..something(((Symbol method, List<dynamic> arguments) {
@@ -1850,7 +1847,7 @@ void main() {
           backgroundCursorColor: Colors.grey,
           controller: controller,
           focusNode: focusNode,
-          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
           cursorColor: Colors.blue,
           autocorrect: true,
           autocorrectionTextRectColor: rectColor,
@@ -1898,7 +1895,7 @@ void main() {
     final TextEditingController controller2 =
         TextEditingController(text: 'Wobble');
     TextEditingController currentController = controller1;
-    StateSetter setState;
+    late StateSetter setState;
 
     final FocusNode focusNode = FocusNode(debugLabel: 'EditableText Focus Node');
     Widget builder() {
@@ -1918,7 +1915,7 @@ void main() {
                       focusNode: focusNode,
                       style: Typography.material2018(platform: TargetPlatform.android)
                           .black
-                          .subtitle1,
+                          .subtitle1!,
                       cursorColor: Colors.blue,
                       selectionControls: materialTextSelectionControls,
                       keyboardType: TextInputType.text,
@@ -2155,7 +2152,7 @@ void main() {
     );
 
     controller.selection =
-        TextSelection.collapsed(offset: controller.text.length);
+        TextSelection.collapsed(offset:controller.text.length);
     await tester.pumpAndSettle();
 
     // At end, can only go backwards.
@@ -2172,7 +2169,7 @@ void main() {
     );
 
     controller.selection =
-        TextSelection.collapsed(offset: controller.text.length - 2);
+        TextSelection.collapsed(offset:controller.text.length - 2);
     await tester.pumpAndSettle();
 
     // Somewhere in the middle, can go in both directions.
@@ -2215,7 +2212,7 @@ void main() {
 
     controller.text = 'test';
     controller.selection =
-        TextSelection.collapsed(offset: controller.text.length);
+        TextSelection.collapsed(offset:controller.text.length);
 
     await tester.pumpWidget(MaterialApp(
       home: EditableText(
@@ -2239,12 +2236,12 @@ void main() {
     );
 
     final RenderEditable render = tester.allRenderObjects.whereType<RenderEditable>().first;
-    final int semanticsId = render.debugSemantics.id;
+    final int semanticsId = render.debugSemantics!.id;
 
     expect(controller.selection.baseOffset, 4);
     expect(controller.selection.extentOffset, 4);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByCharacter, doNotExtendSelection);
     await tester.pumpAndSettle();
 
@@ -2265,13 +2262,13 @@ void main() {
       ),
     );
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByCharacter, doNotExtendSelection);
     await tester.pumpAndSettle();
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByCharacter, doNotExtendSelection);
     await tester.pumpAndSettle();
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByCharacter, doNotExtendSelection);
     await tester.pumpAndSettle();
 
@@ -2291,7 +2288,7 @@ void main() {
       ),
     );
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorForwardByCharacter, doNotExtendSelection);
     await tester.pumpAndSettle();
 
@@ -2307,7 +2304,7 @@ void main() {
 
     controller.text = 'test for words';
     controller.selection =
-    TextSelection.collapsed(offset: controller.text.length);
+    TextSelection.collapsed(offset:controller.text.length);
 
     await tester.pumpWidget(MaterialApp(
       home: EditableText(
@@ -2331,12 +2328,12 @@ void main() {
     );
 
     final RenderEditable render = tester.allRenderObjects.whereType<RenderEditable>().first;
-    final int semanticsId = render.debugSemantics.id;
+    final int semanticsId = render.debugSemantics!.id;
 
     expect(controller.selection.baseOffset, 14);
     expect(controller.selection.extentOffset, 14);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByWord, doNotExtendSelection);
     await tester.pumpAndSettle();
 
@@ -2357,14 +2354,14 @@ void main() {
       ),
     );
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByWord, doNotExtendSelection);
     await tester.pumpAndSettle();
 
     expect(controller.selection.baseOffset, 5);
     expect(controller.selection.extentOffset, 5);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByWord, doNotExtendSelection);
     await tester.pumpAndSettle();
 
@@ -2384,14 +2381,14 @@ void main() {
       ),
     );
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorForwardByWord, doNotExtendSelection);
     await tester.pumpAndSettle();
 
     expect(controller.selection.baseOffset, 5);
     expect(controller.selection.extentOffset, 5);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorForwardByWord, doNotExtendSelection);
     await tester.pumpAndSettle();
 
@@ -2408,7 +2405,7 @@ void main() {
 
     controller.text = 'test';
     controller.selection =
-        TextSelection.collapsed(offset: controller.text.length);
+        TextSelection.collapsed(offset:controller.text.length);
 
     await tester.pumpWidget(MaterialApp(
       home: EditableText(
@@ -2432,12 +2429,12 @@ void main() {
     );
 
     final RenderEditable render = tester.allRenderObjects.whereType<RenderEditable>().first;
-    final int semanticsId = render.debugSemantics.id;
+    final int semanticsId = render.debugSemantics!.id;
 
     expect(controller.selection.baseOffset, 4);
     expect(controller.selection.extentOffset, 4);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByCharacter, extendSelection);
     await tester.pumpAndSettle();
 
@@ -2458,13 +2455,13 @@ void main() {
       ),
     );
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByCharacter, extendSelection);
     await tester.pumpAndSettle();
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByCharacter, extendSelection);
     await tester.pumpAndSettle();
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByCharacter, extendSelection);
     await tester.pumpAndSettle();
 
@@ -2484,14 +2481,14 @@ void main() {
       ),
     );
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorForwardByCharacter, doNotExtendSelection);
     await tester.pumpAndSettle();
 
     expect(controller.selection.baseOffset, 1);
     expect(controller.selection.extentOffset, 1);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorForwardByCharacter, extendSelection);
     await tester.pumpAndSettle();
 
@@ -2508,7 +2505,7 @@ void main() {
 
     controller.text = 'test for words';
     controller.selection =
-    TextSelection.collapsed(offset: controller.text.length);
+    TextSelection.collapsed(offset:controller.text.length);
 
     await tester.pumpWidget(MaterialApp(
       home: EditableText(
@@ -2532,12 +2529,12 @@ void main() {
     );
 
     final RenderEditable render = tester.allRenderObjects.whereType<RenderEditable>().first;
-    final int semanticsId = render.debugSemantics.id;
+    final int semanticsId = render.debugSemantics!.id;
 
     expect(controller.selection.baseOffset, 14);
     expect(controller.selection.extentOffset, 14);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByWord, extendSelection);
     await tester.pumpAndSettle();
 
@@ -2558,14 +2555,14 @@ void main() {
       ),
     );
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByWord, extendSelection);
     await tester.pumpAndSettle();
 
     expect(controller.selection.baseOffset, 14);
     expect(controller.selection.extentOffset, 5);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorBackwardByWord, extendSelection);
     await tester.pumpAndSettle();
 
@@ -2585,14 +2582,14 @@ void main() {
       ),
     );
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorForwardByWord, doNotExtendSelection);
     await tester.pumpAndSettle();
 
     expect(controller.selection.baseOffset, 5);
     expect(controller.selection.extentOffset, 5);
 
-    tester.binding.pipelineOwner.semanticsOwner.performAction(semanticsId,
+    tester.binding.pipelineOwner.semanticsOwner!.performAction(semanticsId,
         SemanticsAction.moveCursorForwardByWord, extendSelection);
     await tester.pumpAndSettle();
 
@@ -2618,7 +2615,7 @@ void main() {
       ),
     ));
 
-    final String expectedValue = '•' * controller.text.length;
+    final String expectedValue = '•' *controller.text.length;
 
     expect(
       semantics,
@@ -2722,7 +2719,7 @@ void main() {
       ),
     ));
 
-    expect(findRenderEditable(tester).text.text, expectedValue);
+    expect(findRenderEditable(tester).text!.text, expectedValue);
 
     expect(
       semantics,
@@ -2786,7 +2783,7 @@ void main() {
     ));
 
     final String expectedValue = obscuringCharacter * originalText.length;
-    expect(findRenderEditable(tester).text.text, expectedValue);
+    expect(findRenderEditable(tester).text!.text, expectedValue);
   });
 
   group('a11y copy/cut/paste', () {
@@ -2803,12 +2800,12 @@ void main() {
       ));
     }
 
-    MockTextSelectionControls controls;
+    late MockTextSelectionControls controls;
 
     setUp(() {
       controller.text = 'test';
       controller.selection =
-          TextSelection.collapsed(offset: controller.text.length);
+          TextSelection.collapsed(offset:controller.text.length);
 
       controls = MockTextSelectionControls();
     });
@@ -2915,7 +2912,7 @@ void main() {
       await tester.tap(find.byType(EditableText));
       await tester.pump();
 
-      final SemanticsOwner owner = tester.binding.pipelineOwner.semanticsOwner;
+      final SemanticsOwner owner = tester.binding.pipelineOwner.semanticsOwner!;
       const int expectedNodeId = 5;
 
       expect(
@@ -2992,7 +2989,7 @@ void main() {
 
     // Simulate selection change via tap to show handles.
     final RenderEditable render = tester.allRenderObjects.whereType<RenderEditable>().first;
-    expect(render.text.style.fontStyle, FontStyle.italic);
+    expect(render.text!.style!.fontStyle, FontStyle.italic);
   });
 
   testWidgets('Formatters are skipped if text has not changed', (WidgetTester tester) async {
@@ -3058,7 +3055,7 @@ void main() {
           child: EditableText(
             controller: controller,
             focusNode: FocusNode(),
-            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
             cursorColor: Colors.blue,
             backgroundCursorColor: Colors.grey,
           ),
@@ -3089,7 +3086,7 @@ void main() {
           child: EditableText(
             controller: controller,
             focusNode: FocusNode(),
-            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
             cursorColor: Colors.blue,
             backgroundCursorColor: Colors.grey,
           ),
@@ -3135,7 +3132,7 @@ void main() {
                 key: ValueKey<String>(controller1.text),
                 controller: controller1,
                 focusNode: FocusNode(),
-                style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+                style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
                 cursorColor: Colors.blue,
                 backgroundCursorColor: Colors.grey,
               ),
@@ -3144,7 +3141,7 @@ void main() {
                 key: ValueKey<String>(controller2.text),
                 controller: controller2,
                 focusNode: FocusNode(),
-                style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+                style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
                 cursorColor: Colors.blue,
                 backgroundCursorColor: Colors.grey,
                 minLines: 10,
@@ -3280,7 +3277,7 @@ void main() {
 
   testWidgets('text styling info is sent on style update', (WidgetTester tester) async {
     final GlobalKey<EditableTextState> editableTextKey = GlobalKey<EditableTextState>();
-    StateSetter setState;
+    late StateSetter setState;
     const TextStyle textStyle1 = TextStyle(
       fontSize: 20.0,
       fontFamily: 'RobotoMono',
@@ -3466,7 +3463,7 @@ void main() {
           child: EditableText(
             controller: controller,
             focusNode: FocusNode(),
-            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
             cursorColor: Colors.blue,
             backgroundCursorColor: Colors.grey,
             keyboardAppearance: Brightness.dark,
@@ -3511,18 +3508,18 @@ void main() {
 
     final RenderEditable renderEditable = findRenderEditable(tester);
     // The actual text span is split into 3 parts with the middle part underlined.
-    expect(renderEditable.text.children.length, 3);
-    final TextSpan textSpan = renderEditable.text.children[1] as TextSpan;
+    expect(renderEditable.text!.children!.length, 3);
+    final TextSpan textSpan = renderEditable.text!.children![1] as TextSpan;
     expect(textSpan.text, 'composing');
-    expect(textSpan.style.decoration, TextDecoration.underline);
+    expect(textSpan.style!.decoration, TextDecoration.underline);
 
     focusNode.unfocus();
     await tester.pump();
 
-    expect(renderEditable.text.children, isNull);
+    expect(renderEditable.text!.children, isNull);
     // Everything's just formated the same way now.
-    expect(renderEditable.text.text, 'text composing text');
-    expect(renderEditable.text.style.decoration, isNull);
+    expect(renderEditable.text!.text, 'text composing text');
+    expect(renderEditable.text!.style!.decoration, isNull);
   });
 
   testWidgets('text selection handle visibility', (WidgetTester tester) async {
@@ -3539,7 +3536,7 @@ void main() {
             showSelectionHandles: true,
             controller: controller,
             focusNode: FocusNode(),
-            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
             cursorColor: Colors.blue,
             backgroundCursorColor: Colors.grey,
             selectionControls: materialTextSelectionControls,
@@ -3650,7 +3647,7 @@ void main() {
             throw TestFailure("HandlePositionInViewport can't be null.");
         }
       }
-      expect(state.selectionOverlay.handlesAreVisible, isTrue);
+      expect(state.selectionOverlay!.handlesAreVisible, isTrue);
       testPosition(handles[0].localToGlobal(Offset.zero).dx, leftPosition);
       testPosition(handles[1].localToGlobal(Offset.zero).dx, rightPosition);
     }
@@ -3663,13 +3660,13 @@ void main() {
 
     // Drag the text slightly so the first word is partially visible. Only the
     // right handle should be visible.
-    scrollable.controller.jumpTo(20.0);
+    scrollable.controller!.jumpTo(20.0);
     await verifyVisibility(HandlePositionInViewport.leftEdge, false, HandlePositionInViewport.within, true);
 
     // Drag the text all the way to the left so the first word is not visible at
     // all (and the second word is fully visible). Both handles should be
     // invisible now.
-    scrollable.controller.jumpTo(200.0);
+    scrollable.controller!.jumpTo(200.0);
     await verifyVisibility(HandlePositionInViewport.leftEdge, false, HandlePositionInViewport.leftEdge, false);
 
     // Tap to unselect.
@@ -3684,12 +3681,12 @@ void main() {
 
     // Drag the text slightly to the right. Only the left handle should be
     // visible.
-    scrollable.controller.jumpTo(150);
+    scrollable.controller!.jumpTo(150);
     await verifyVisibility(HandlePositionInViewport.within, true, HandlePositionInViewport.rightEdge, false);
 
     // Drag the text all the way to the right, so the second word is not visible
     // at all. Again, both handles should be invisible.
-    scrollable.controller.jumpTo(0);
+    scrollable.controller!.jumpTo(0);
     await verifyVisibility(HandlePositionInViewport.rightEdge, false, HandlePositionInViewport.rightEdge, false);
 
     // On web, we don't show the Flutter toolbar and instead rely on the browser
@@ -3710,7 +3707,7 @@ void main() {
             controller: controller,
             showSelectionHandles: true,
             focusNode: FocusNode(),
-            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
             cursorColor: Colors.blue,
             backgroundCursorColor: Colors.grey,
             selectionControls: materialTextSelectionControls,
@@ -3750,7 +3747,7 @@ void main() {
         70.0 + kMinInteractiveDimension,
       ),
     );
-    expect(state.selectionOverlay.handlesAreVisible, isTrue);
+    expect(state.selectionOverlay!.handlesAreVisible, isTrue);
     expect(controller.selection.base.offset, 0);
     expect(controller.selection.extent.offset, 5);
 
@@ -3770,7 +3767,7 @@ void main() {
         bool wordModifier = false,
         bool lineModifier = false,
         bool shortcutModifier = false,
-        String platform,
+        required String platform,
       }) async {
     if (shift) {
       await tester.sendKeyDownEvent(LogicalKeyboardKey.shiftLeft, platform: platform);
@@ -3817,15 +3814,15 @@ void main() {
     }
   }
 
-  Future<void> testTextEditing(WidgetTester tester, {String platform}) async {
+  Future<void> testTextEditing(WidgetTester tester, {required String platform}) async {
     final TextEditingController controller = TextEditingController(text: testText);
     controller.selection = const TextSelection(
       baseOffset: 0,
       extentOffset: 0,
       affinity: TextAffinity.upstream,
     );
-    TextSelection selection;
-    SelectionChangedCause cause;
+    late TextSelection selection;
+    late SelectionChangedCause cause;
     await tester.pumpWidget(MaterialApp(
       home: Align(
         alignment: Alignment.topLeft,
@@ -3837,15 +3834,15 @@ void main() {
             showSelectionHandles: true,
             autofocus: true,
             focusNode: FocusNode(),
-            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
             cursorColor: Colors.blue,
             backgroundCursorColor: Colors.grey,
             selectionControls: materialTextSelectionControls,
             keyboardType: TextInputType.text,
             textAlign: TextAlign.right,
-            onSelectionChanged: (TextSelection newSelection, SelectionChangedCause newCause) {
+            onSelectionChanged: (TextSelection newSelection, SelectionChangedCause? newCause) {
               selection = newSelection;
-              cause = newCause;
+              cause = newCause!;
             },
           ),
         ),
@@ -4275,7 +4272,7 @@ void main() {
       reason: 'on $platform',
     );
     expect(
-      (await Clipboard.getData(Clipboard.kTextPlain)).text,
+      (await Clipboard.getData(Clipboard.kTextPlain))!.text,
       equals('is the'),
       reason: 'on $platform',
     );
@@ -4326,7 +4323,7 @@ void main() {
       reason: 'on $platform',
     );
     expect(controller.text, equals(testText), reason: 'on $platform');
-    expect((await Clipboard.getData(Clipboard.kTextPlain)).text, equals(testText));
+    expect((await Clipboard.getData(Clipboard.kTextPlain))!.text, equals(testText));
 
     // Delete
     await sendKeys(
@@ -4386,7 +4383,7 @@ void main() {
               showSelectionHandles: true,
               controller: controller,
               focusNode: FocusNode(),
-              style: Typography.material2018(platform: TargetPlatform.iOS).black.subtitle1,
+              style: Typography.material2018(platform: TargetPlatform.iOS).black.subtitle1!,
               cursorColor: Colors.blue,
               backgroundCursorColor: Colors.grey,
               selectionControls: cupertinoTextSelectionControls,
@@ -4495,7 +4492,7 @@ void main() {
             throw TestFailure("HandlePositionInViewport can't be null.");
         }
       }
-      expect(state.selectionOverlay.handlesAreVisible, isTrue);
+      expect(state.selectionOverlay!.handlesAreVisible, isTrue);
       testPosition(handles[0].localToGlobal(Offset.zero).dx, leftPosition);
       testPosition(handles[1].localToGlobal(Offset.zero).dx, rightPosition);
     }
@@ -4508,13 +4505,13 @@ void main() {
 
     // Drag the text slightly so the first word is partially visible. Only the
     // right handle should be visible.
-    scrollable.controller.jumpTo(20.0);
+    scrollable.controller!.jumpTo(20.0);
     await verifyVisibility(HandlePositionInViewport.leftEdge, false, HandlePositionInViewport.within, true);
 
     // Drag the text all the way to the left so the first word is not visible at
     // all (and the second word is fully visible). Both handles should be
     // invisible now.
-    scrollable.controller.jumpTo(200.0);
+    scrollable.controller!.jumpTo(200.0);
     await verifyVisibility(HandlePositionInViewport.leftEdge, false, HandlePositionInViewport.leftEdge, false);
 
     // Tap to unselect.
@@ -4529,12 +4526,12 @@ void main() {
 
     // Drag the text slightly to the right. Only the left handle should be
     // visible.
-    scrollable.controller.jumpTo(150);
+    scrollable.controller!.jumpTo(150);
     await verifyVisibility(HandlePositionInViewport.within, true, HandlePositionInViewport.rightEdge, false);
 
     // Drag the text all the way to the right, so the second word is not visible
     // at all. Again, both handles should be invisible.
-    scrollable.controller.jumpTo(0);
+    scrollable.controller!.jumpTo(0);
     await verifyVisibility(HandlePositionInViewport.rightEdge, false, HandlePositionInViewport.rightEdge, false);
 
     // On web, we don't show the Flutter toolbar and instead rely on the browser
@@ -4556,7 +4553,7 @@ void main() {
             maxLines: 2,
             controller: controller,
             focusNode: FocusNode(),
-            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1.copyWith(fontFamily: 'Roboto'),
+            style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!.copyWith(fontFamily: 'Roboto'),
             cursorColor: Colors.blue,
             backgroundCursorColor: Colors.grey,
             selectionControls: materialTextSelectionControls,
@@ -4571,24 +4568,24 @@ void main() {
     final RenderEditable renderEditable = state.renderEditable;
     final Scrollable scrollable = tester.widget<Scrollable>(find.byType(Scrollable));
 
-    expect(scrollable.controller.position.viewportDimension, equals(28));
-    expect(scrollable.controller.position.pixels, equals(0));
+    expect(scrollable.controller!.position.viewportDimension, equals(28));
+    expect(scrollable.controller!.position.pixels, equals(0));
 
     expect(renderEditable.maxScrollExtent, equals(14));
 
-    scrollable.controller.jumpTo(20.0);
+    scrollable.controller!.jumpTo(20.0);
     await tester.pump();
-    expect(scrollable.controller.position.pixels, equals(20));
+    expect(scrollable.controller!.position.pixels, equals(20));
 
     state.bringIntoView(const TextPosition(offset: 0));
     await tester.pump();
-    expect(scrollable.controller.position.pixels, equals(0));
+    expect(scrollable.controller!.position.pixels, equals(0));
 
 
     state.bringIntoView(const TextPosition(offset: 13));
     await tester.pump();
-    expect(scrollable.controller.position.pixels, equals(14));
-    expect(scrollable.controller.position.pixels, equals(renderEditable.maxScrollExtent));
+    expect(scrollable.controller!.position.pixels, equals(14));
+    expect(scrollable.controller!.position.pixels, equals(renderEditable.maxScrollExtent));
   });
 
   testWidgets('bringIntoView brings the caret into view when in a viewport', (WidgetTester tester) async {
@@ -4637,7 +4634,7 @@ void main() {
     final TextEditingController controller = TextEditingController(text: testText * 20);
     final ScrollController scrollController = ScrollController();
 
-    Future<void> buildWithPhysics({ ScrollPhysics physics }) async {
+    Future<void> buildWithPhysics({ ScrollPhysics? physics }) async {
       await tester.pumpWidget(MaterialApp(
         home: Align(
           alignment: Alignment.topLeft,
@@ -4746,7 +4743,7 @@ void main() {
       focusNode: FocusNode(),
       cursorColor: Colors.red,
       backgroundCursorColor: Colors.blue,
-      style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1.copyWith(fontFamily: 'Roboto'),
+      style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!.copyWith(fontFamily: 'Roboto'),
       keyboardType: TextInputType.text,
     );
 
@@ -4789,7 +4786,7 @@ void main() {
       focusNode: FocusNode(),
       cursorColor: Colors.red,
       backgroundCursorColor: Colors.blue,
-      style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1.copyWith(fontFamily: 'Roboto'),
+      style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!.copyWith(fontFamily: 'Roboto'),
       keyboardType: TextInputType.text,
     );
 
@@ -4821,7 +4818,7 @@ void main() {
       expect(m.method, logOrder[index]);
       index++;
     }
-    expect(tester.testTextInput.editingState['text'], 'flutter is the best!');
+    expect(tester.testTextInput.editingState!['text'], 'flutter is the best!');
   });
 
   testWidgets('setEditingState is called when text changes on controller', (WidgetTester tester) async {
@@ -4835,7 +4832,7 @@ void main() {
       focusNode: FocusNode(),
       cursorColor: Colors.red,
       backgroundCursorColor: Colors.blue,
-      style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1.copyWith(fontFamily: 'Roboto'),
+      style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!.copyWith(fontFamily: 'Roboto'),
       keyboardType: TextInputType.text,
     );
 
@@ -4869,7 +4866,7 @@ void main() {
       tester.testTextInput.log.map((MethodCall m) => m.method),
       logOrder,
     );
-    expect(tester.testTextInput.editingState['text'], 'flutter is the best!...');
+    expect(tester.testTextInput.editingState!['text'], 'flutter is the best!...');
   });
 
   testWidgets('Synchronous test of local and remote editing values', (WidgetTester tester) async {
@@ -4885,7 +4882,7 @@ void main() {
       return newValue;
     });
     final TextEditingController controller = TextEditingController();
-    StateSetter setState;
+    late StateSetter setState;
 
     final FocusNode focusNode = FocusNode(debugLabel: 'EditableText Focus Node');
     Widget builder() {
@@ -5174,7 +5171,7 @@ void main() {
         showSelectionHandles: true,
         autofocus: true,
         focusNode: FocusNode(),
-        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
         cursorColor: Colors.blue,
         backgroundCursorColor: Colors.grey,
         selectionControls: materialTextSelectionControls,
@@ -5221,7 +5218,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals(''));
+    expect(tester.testTextInput.editingState!['text'], equals(''));
     expect(state.wantKeepAlive, true);
 
     state.updateEditingValue(const TextEditingValue(text: ''));
@@ -5294,7 +5291,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals(''));
+    expect(tester.testTextInput.editingState!['text'], equals(''));
     expect(state.wantKeepAlive, true);
 
     // We no longer perform full repeat filtering in framework, it is now left
@@ -5373,7 +5370,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals('test'));
+    expect(tester.testTextInput.editingState!['text'], equals('test'));
     expect(state.wantKeepAlive, true);
 
     expect(formatter.formatCallCount, 0);
@@ -5414,7 +5411,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals(''));
+    expect(tester.testTextInput.editingState!['text'], equals(''));
     expect(state.wantKeepAlive, true);
 
     // Simple mixed directional input.
@@ -5469,7 +5466,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals(''));
+    expect(tester.testTextInput.editingState!['text'], equals(''));
     expect(state.wantKeepAlive, true);
 
     // Does not overwrite existing RLM or LRM characters
@@ -5517,7 +5514,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals(''));
+    expect(tester.testTextInput.editingState!['text'], equals(''));
     expect(state.wantKeepAlive, true);
 
     // Can be passed through formatter repeatedly without leaking/growing.
@@ -5559,7 +5556,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals(''));
+    expect(tester.testTextInput.editingState!['text'], equals(''));
     expect(state.wantKeepAlive, true);
 
     // Doesn't eat emojis
@@ -5599,7 +5596,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals(''));
+    expect(tester.testTextInput.editingState!['text'], equals(''));
     expect(state.wantKeepAlive, true);
 
     // Doesn't eat emojis
@@ -5639,7 +5636,7 @@ void main() {
 
     final EditableTextState state =
         tester.state<EditableTextState>(find.byType(EditableText));
-    expect(tester.testTextInput.editingState['text'], equals(''));
+    expect(tester.testTextInput.editingState!['text'], equals(''));
     expect(state.wantKeepAlive, true);
 
     // Simulate deleting only the trailing RTL mark.
@@ -5679,7 +5676,7 @@ void main() {
 
     await tester.pump();
 
-    expect(RendererBinding.instance.mouseTracker.debugDeviceActiveCursor(1), SystemMouseCursors.click);
+    expect(RendererBinding.instance!.mouseTracker.debugDeviceActiveCursor(1), SystemMouseCursors.click);
 
     // Test default cursor
     await tester.pumpWidget(
@@ -5704,17 +5701,17 @@ void main() {
       ),
     );
 
-    expect(RendererBinding.instance.mouseTracker.debugDeviceActiveCursor(1), SystemMouseCursors.text);
+    expect(RendererBinding.instance!.mouseTracker.debugDeviceActiveCursor(1), SystemMouseCursors.text);
   });
 
   testWidgets('Can access characters on editing string', (WidgetTester tester) async {
-    int charactersLength;
+    late int charactersLength;
     final Widget widget = MaterialApp(
       home: EditableText(
         backgroundCursorColor: Colors.grey,
         controller: TextEditingController(),
         focusNode: FocusNode(),
-        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+        style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
         cursorColor: Colors.blue,
         selectionControls: materialTextSelectionControls,
         keyboardType: TextInputType.text,
@@ -6065,10 +6062,10 @@ void main() {
           focusNode: FocusNode(),
           cursorColor: Colors.red,
           backgroundCursorColor: Colors.blue,
-          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1.copyWith(fontFamily: 'Roboto'),
+          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!.copyWith(fontFamily: 'Roboto'),
           keyboardType: TextInputType.text,
           selectionControls: materialTextSelectionControls,
-          onSelectionChanged: (TextSelection selection, SelectionChangedCause cause) {
+          onSelectionChanged: (TextSelection selection, SelectionChangedCause? cause) {
             throw FlutterError(errorText);
           },
         ),
@@ -6092,7 +6089,7 @@ void main() {
           focusNode: FocusNode(),
           cursorColor: Colors.red,
           backgroundCursorColor: Colors.blue,
-          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1.copyWith(fontFamily: 'Roboto'),
+          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!.copyWith(fontFamily: 'Roboto'),
           keyboardType: TextInputType.text,
           onChanged: (String text) {
             throw FlutterError(errorText);
@@ -6118,7 +6115,7 @@ void main() {
           focusNode: FocusNode(),
           cursorColor: Colors.red,
           backgroundCursorColor: Colors.blue,
-          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1.copyWith(fontFamily: 'Roboto'),
+          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!.copyWith(fontFamily: 'Roboto'),
           keyboardType: TextInputType.text,
           onEditingComplete: () {
             throw FlutterError(errorText);
@@ -6149,7 +6146,7 @@ void main() {
           focusNode: FocusNode(),
           cursorColor: Colors.red,
           backgroundCursorColor: Colors.blue,
-          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1.copyWith(fontFamily: 'Roboto'),
+          style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!.copyWith(fontFamily: 'Roboto'),
           keyboardType: TextInputType.text,
           onSubmitted: (String text) {
             throw FlutterError(errorText);
@@ -6176,8 +6173,8 @@ class MockTextFormatter extends TextInputFormatter {
 
   int formatCallCount;
   List<String> log;
-  TextEditingValue lastOldValue;
-  TextEditingValue lastNewValue;
+  late TextEditingValue lastOldValue;
+  late TextEditingValue lastNewValue;
 
   @override
   TextEditingValue formatEditUpdate(
@@ -6242,7 +6239,7 @@ class MockTextSelectionControls extends Fake implements TextSelectionControls {
   int copyCount = 0;
 
   @override
-  void handleCopy(TextSelectionDelegate delegate, ClipboardStatusNotifier clipboardStatus) {
+  void handleCopy(TextSelectionDelegate delegate, ClipboardStatusNotifier? clipboardStatus) {
     copyCount += 1;
   }
 
@@ -6274,11 +6271,11 @@ class MockTextSelectionControls extends Fake implements TextSelectionControls {
 
 class CustomStyleEditableText extends EditableText {
   CustomStyleEditableText({
-    Key key,
-    TextEditingController controller,
-    Color cursorColor,
-    FocusNode focusNode,
-    TextStyle style,
+    Key? key,
+    required TextEditingController controller,
+    required Color cursorColor,
+    required FocusNode focusNode,
+    required TextStyle style,
   }) : super(
           key: key,
           controller: controller,
@@ -6304,9 +6301,9 @@ class CustomStyleEditableTextState extends EditableTextState {
 
 class TransformedEditableText extends StatefulWidget {
   const TransformedEditableText({
-    Key key,
-    this.offset,
-    this.transformButtonKey,
+    Key? key,
+    required this.offset,
+    required this.transformButtonKey,
   }) : super(key: key);
 
   final Offset offset;
@@ -6336,7 +6333,7 @@ class _TransformedEditableTextState extends State<TransformedEditableText> {
               child: EditableText(
                 controller: TextEditingController(),
                 focusNode: FocusNode(),
-                style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1,
+                style: Typography.material2018(platform: TargetPlatform.android).black.subtitle1!,
                 cursorColor: Colors.blue,
                 backgroundCursorColor: Colors.grey,
               ),
@@ -6358,19 +6355,19 @@ class _TransformedEditableTextState extends State<TransformedEditableText> {
 }
 
 class NoImplicitScrollPhysics extends AlwaysScrollableScrollPhysics {
-  const NoImplicitScrollPhysics({ ScrollPhysics parent }) : super(parent: parent);
+  const NoImplicitScrollPhysics({ ScrollPhysics? parent }) : super(parent: parent);
 
   @override
   bool get allowImplicitScrolling => false;
 
   @override
-  NoImplicitScrollPhysics applyTo(ScrollPhysics ancestor) {
-    return NoImplicitScrollPhysics(parent: buildParent(ancestor));
+  NoImplicitScrollPhysics applyTo(ScrollPhysics? ancestor) {
+    return NoImplicitScrollPhysics(parent: buildParent(ancestor)!);
   }
 }
 
 class SkipPainting extends SingleChildRenderObjectWidget {
-  const SkipPainting({ Key key, Widget child }): super(key: key, child: child);
+  const SkipPainting({ Key? key, required Widget child }): super(key: key, child: child);
 
   @override
   SkipPaintingRenderObject createRenderObject(BuildContext context) => SkipPaintingRenderObject();
diff --git a/packages/flutter/test/widgets/ensure_visible_test.dart b/packages/flutter/test/widgets/ensure_visible_test.dart
index 1a17b01612..390bad4f90 100644
--- a/packages/flutter/test/widgets/ensure_visible_test.dart
+++ b/packages/flutter/test/widgets/ensure_visible_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:math' as math;
 
 import 'package:flutter_test/flutter_test.dart';
diff --git a/packages/flutter/test/widgets/error_widget_builder_test.dart b/packages/flutter/test/widgets/error_widget_builder_test.dart
index 377ec23bb6..5126680529 100644
--- a/packages/flutter/test/widgets/error_widget_builder_test.dart
+++ b/packages/flutter/test/widgets/error_widget_builder_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
diff --git a/packages/flutter/test/widgets/fade_in_image_test.dart b/packages/flutter/test/widgets/fade_in_image_test.dart
index 371879e78d..abc1c7e403 100644
--- a/packages/flutter/test/widgets/fade_in_image_test.dart
+++ b/packages/flutter/test/widgets/fade_in_image_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:typed_data';
 import 'dart:ui' as ui;
 
@@ -22,21 +20,21 @@ class FadeInImageParts {
         assert(target != null);
 
   final ComponentElement fadeInImageElement;
-  final FadeInImageElements placeholder;
+  final FadeInImageElements? placeholder;
   final FadeInImageElements target;
 
-  State get state {
-    StatefulElement animatedFadeOutFadeInElement;
+  State? get state {
+    StatefulElement? animatedFadeOutFadeInElement;
     fadeInImageElement.visitChildren((Element child) {
       expect(animatedFadeOutFadeInElement, isNull);
       animatedFadeOutFadeInElement = child as StatefulElement;
     });
     expect(animatedFadeOutFadeInElement, isNotNull);
-    return animatedFadeOutFadeInElement.state;
+    return animatedFadeOutFadeInElement!.state;
   }
 
-  Element get semanticsElement {
-    Element result;
+  Element? get semanticsElement {
+    Element? result;
     fadeInImageElement.visitChildren((Element child) {
       if (child.widget is Semantics)
         result = child;
@@ -49,39 +47,39 @@ class FadeInImageElements {
   const FadeInImageElements(this.rawImageElement, this.fadeTransitionElement);
 
   final Element rawImageElement;
-  final Element fadeTransitionElement;
+  final Element? fadeTransitionElement;
 
   RawImage get rawImage => rawImageElement.widget as RawImage;
-  FadeTransition get fadeTransition => fadeTransitionElement?.widget as FadeTransition;
-  double get opacity => fadeTransition == null ? 1 : fadeTransition.opacity.value;
+  FadeTransition? get fadeTransition => fadeTransitionElement?.widget as FadeTransition?;
+  double get opacity => fadeTransition == null ? 1 : fadeTransition!.opacity.value;
 }
 
-class LoadTestImageProvider extends ImageProvider<dynamic> {
+class LoadTestImageProvider extends ImageProvider<Object> {
   LoadTestImageProvider(this.provider);
 
   final ImageProvider provider;
 
-  ImageStreamCompleter testLoad(dynamic key, DecoderCallback decode) {
+  ImageStreamCompleter testLoad(Object key, DecoderCallback decode) {
     return provider.load(key, decode);
   }
 
   @override
-  Future<dynamic> obtainKey(ImageConfiguration configuration) {
-    return null;
+  Future<Object> obtainKey(ImageConfiguration configuration) {
+    throw UnimplementedError();
   }
 
   @override
-  ImageStreamCompleter load(dynamic key, DecoderCallback decode) {
-    return null;
+  ImageStreamCompleter load(Object key, DecoderCallback decode) {
+    throw UnimplementedError();
   }
 }
 
 FadeInImageParts findFadeInImage(WidgetTester tester) {
   final List<FadeInImageElements> elements = <FadeInImageElements>[];
   final Iterable<Element> rawImageElements = tester.elementList(find.byType(RawImage));
-  ComponentElement fadeInImageElement;
+  ComponentElement? fadeInImageElement;
   for (final Element rawImageElement in rawImageElements) {
-    Element fadeTransitionElement;
+    Element? fadeTransitionElement;
     rawImageElement.visitAncestorElements((Element ancestor) {
       if (ancestor.widget is FadeTransition) {
         fadeTransitionElement = ancestor;
@@ -99,10 +97,10 @@ FadeInImageParts findFadeInImage(WidgetTester tester) {
     elements.add(FadeInImageElements(rawImageElement, fadeTransitionElement));
   }
   if (elements.length == 2) {
-    return FadeInImageParts(fadeInImageElement, elements.last, elements.first);
+    return FadeInImageParts(fadeInImageElement!, elements.last, elements.first);
   } else {
     expect(elements, hasLength(1));
-    return FadeInImageParts(fadeInImageElement, null, elements.first);
+    return FadeInImageParts(fadeInImageElement!, null, elements.first);
   }
 }
 
@@ -127,30 +125,30 @@ Future<void> main() async {
         excludeFromSemantics: true,
       ));
 
-      expect(findFadeInImage(tester).placeholder.rawImage.image, null);
+      expect(findFadeInImage(tester).placeholder!.rawImage.image, null);
       expect(findFadeInImage(tester).target.rawImage.image, null);
 
       placeholderProvider.complete();
       await tester.pump();
-      expect(findFadeInImage(tester).placeholder.rawImage.image.isCloneOf(placeholderImage), true);
+      expect(findFadeInImage(tester).placeholder!.rawImage.image!.isCloneOf(placeholderImage), true);
       expect(findFadeInImage(tester).target.rawImage.image, null);
 
       imageProvider.complete();
       await tester.pump();
       for (int i = 0; i < 5; i += 1) {
         final FadeInImageParts parts = findFadeInImage(tester);
-        expect(parts.placeholder.rawImage.image.isCloneOf(placeholderImage), true);
-        expect(parts.target.rawImage.image.isCloneOf(targetImage), true);
-        expect(parts.placeholder.opacity, moreOrLessEquals(1 - i / 5));
+        expect(parts.placeholder!.rawImage.image!.isCloneOf(placeholderImage), true);
+        expect(parts.target.rawImage.image!.isCloneOf(targetImage), true);
+        expect(parts.placeholder!.opacity, moreOrLessEquals(1 - i / 5));
         expect(parts.target.opacity, 0);
         await tester.pump(const Duration(milliseconds: 10));
       }
 
       for (int i = 0; i < 5; i += 1) {
         final FadeInImageParts parts = findFadeInImage(tester);
-        expect(parts.placeholder.rawImage.image.isCloneOf(placeholderImage), true);
-        expect(parts.target.rawImage.image.isCloneOf(targetImage), true);
-        expect(parts.placeholder.opacity, 0);
+        expect(parts.placeholder!.rawImage.image!.isCloneOf(placeholderImage), true);
+        expect(parts.target.rawImage.image!.isCloneOf(targetImage), true);
+        expect(parts.placeholder!.opacity, 0);
         expect(parts.target.opacity, moreOrLessEquals(i / 5));
         await tester.pump(const Duration(milliseconds: 10));
       }
@@ -159,7 +157,7 @@ Future<void> main() async {
         placeholder: placeholderProvider,
         image: imageProvider,
       ));
-      expect(findFadeInImage(tester).target.rawImage.image.isCloneOf(targetImage), true);
+      expect(findFadeInImage(tester).target.rawImage.image!.isCloneOf(targetImage), true);
       expect(findFadeInImage(tester).target.opacity, 1);
     });
 
@@ -174,7 +172,7 @@ Future<void> main() async {
         image: imageProvider,
       ));
 
-      expect(findFadeInImage(tester).target.rawImage.image.isCloneOf(targetImage), true);
+      expect(findFadeInImage(tester).target.rawImage.image!.isCloneOf(targetImage), true);
       expect(findFadeInImage(tester).placeholder, isNull);
       expect(findFadeInImage(tester).target.opacity, 1);
     });
@@ -192,10 +190,10 @@ Future<void> main() async {
         excludeFromSemantics: true,
       ));
 
-      final State state = findFadeInImage(tester).state;
+      final State? state = findFadeInImage(tester).state;
       placeholderProvider.complete();
       await tester.pump();
-      expect(findFadeInImage(tester).placeholder.rawImage.image.isCloneOf(placeholderImage), true);
+      expect(findFadeInImage(tester).placeholder!.rawImage.image!.isCloneOf(placeholderImage), true);
 
       await tester.pumpWidget(FadeInImage(
         placeholder: secondPlaceholderProvider,
@@ -207,7 +205,7 @@ Future<void> main() async {
 
       secondPlaceholderProvider.complete();
       await tester.pump();
-      expect(findFadeInImage(tester).placeholder.rawImage.image.isCloneOf(replacementImage), true);
+      expect(findFadeInImage(tester).placeholder!.rawImage.image!.isCloneOf(replacementImage), true);
       expect(findFadeInImage(tester).state, same(state));
     });
 
@@ -246,7 +244,7 @@ Future<void> main() async {
         excludeFromSemantics: true,
       ));
 
-      final State state = findFadeInImage(tester).state;
+      final State? state = findFadeInImage(tester).state;
       placeholderProvider.complete();
       imageProvider.complete();
       await tester.pump();
@@ -263,15 +261,15 @@ Future<void> main() async {
       secondImageProvider.complete();
       await tester.pump();
 
-      expect(findFadeInImage(tester).target.rawImage.image.isCloneOf(replacementImage), true);
+      expect(findFadeInImage(tester).target.rawImage.image!.isCloneOf(replacementImage), true);
       expect(findFadeInImage(tester).state, same(state));
-      expect(findFadeInImage(tester).placeholder.opacity, moreOrLessEquals(1));
+      expect(findFadeInImage(tester).placeholder!.opacity, moreOrLessEquals(1));
       expect(findFadeInImage(tester).target.opacity, moreOrLessEquals(0));
       await tester.pump(animationDuration);
-      expect(findFadeInImage(tester).placeholder.opacity, moreOrLessEquals(0));
+      expect(findFadeInImage(tester).placeholder!.opacity, moreOrLessEquals(0));
       expect(findFadeInImage(tester).target.opacity, moreOrLessEquals(0));
       await tester.pump(animationDuration);
-      expect(findFadeInImage(tester).placeholder.opacity, moreOrLessEquals(0));
+      expect(findFadeInImage(tester).placeholder!.opacity, moreOrLessEquals(0));
       expect(findFadeInImage(tester).target.opacity, moreOrLessEquals(1));
     });
 
@@ -287,7 +285,7 @@ Future<void> main() async {
         excludeFromSemantics: true,
       ));
 
-      final State state = findFadeInImage(tester).state;
+      final State? state = findFadeInImage(tester).state;
       placeholderProvider.complete();
       imageProvider.complete();
       await tester.pump();
@@ -302,10 +300,10 @@ Future<void> main() async {
       ));
 
       expect(findFadeInImage(tester).state, same(state));
-      expect(findFadeInImage(tester).placeholder.opacity, moreOrLessEquals(0));
+      expect(findFadeInImage(tester).placeholder!.opacity, moreOrLessEquals(0));
       expect(findFadeInImage(tester).target.opacity, moreOrLessEquals(0));
       await tester.pump(animationDuration);
-      expect(findFadeInImage(tester).placeholder.opacity, moreOrLessEquals(0));
+      expect(findFadeInImage(tester).placeholder!.opacity, moreOrLessEquals(0));
       expect(findFadeInImage(tester).target.opacity, moreOrLessEquals(1));
     });
 
@@ -323,12 +321,12 @@ Future<void> main() async {
         );
 
         bool called = false;
-        final DecoderCallback decode = (Uint8List bytes, {int cacheWidth, int cacheHeight, bool allowUpscaling}) {
+        final DecoderCallback decode = (Uint8List bytes, {int? cacheWidth, int? cacheHeight, bool allowUpscaling = false}) {
           expect(cacheWidth, 20);
           expect(cacheHeight, 30);
           expect(allowUpscaling, false);
           called = true;
-          return PaintingBinding.instance.instantiateImageCodec(bytes, cacheWidth: cacheWidth, cacheHeight: cacheHeight, allowUpscaling: allowUpscaling);
+          return PaintingBinding.instance!.instantiateImageCodec(bytes, cacheWidth: cacheWidth, cacheHeight: cacheHeight, allowUpscaling: allowUpscaling);
         };
         final ImageProvider resizeImage = image.placeholder;
         expect(image.placeholder, isA<ResizeImage>());
@@ -346,12 +344,12 @@ Future<void> main() async {
         );
 
         bool called = false;
-        final DecoderCallback decode = (Uint8List bytes, {int cacheWidth, int cacheHeight, bool allowUpscaling}) {
+        final DecoderCallback decode = (Uint8List bytes, {int? cacheWidth, int? cacheHeight, bool allowUpscaling = false}) {
           expect(cacheWidth, null);
           expect(cacheHeight, null);
-          expect(allowUpscaling, null);
+          expect(allowUpscaling, false);
           called = true;
-          return PaintingBinding.instance.instantiateImageCodec(bytes, cacheWidth: cacheWidth, cacheHeight: cacheHeight);
+          return PaintingBinding.instance!.instantiateImageCodec(bytes, cacheWidth: cacheWidth, cacheHeight: cacheHeight);
         };
         // image.placeholder should be an instance of MemoryImage instead of ResizeImage
         final ImageProvider memoryImage = image.placeholder;
diff --git a/packages/flutter/test/widgets/fade_transition_test.dart b/packages/flutter/test/widgets/fade_transition_test.dart
index 08581252f3..edcfce8630 100644
--- a/packages/flutter/test/widgets/fade_transition_test.dart
+++ b/packages/flutter/test/widgets/fade_transition_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -12,8 +10,8 @@ void main() {
   testWidgets('FadeTransition', (WidgetTester tester) async {
     final DebugPrintCallback oldPrint = debugPrint;
     final List<String> log = <String>[];
-    debugPrint = (String message, { int wrapWidth }) {
-      log.add(message);
+    debugPrint = (String? message, { int? wrapWidth }) {
+      log.add(message!);
     };
     debugPrintBuildScope = true;
     final AnimationController controller = AnimationController(
diff --git a/packages/flutter/test/widgets/fitted_box_test.dart b/packages/flutter/test/widgets/fitted_box_test.dart
index 92fe6d099b..7f5c979fda 100644
--- a/packages/flutter/test/widgets/fitted_box_test.dart
+++ b/packages/flutter/test/widgets/fitted_box_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -595,12 +593,11 @@ void main() {
 
 List<Type> getLayers() {
   final List<Type> layers = <Type>[];
-  Layer layer = RendererBinding.instance.renderView.debugLayer;
-  while (layer is ContainerLayer) {
-    final ContainerLayer container = layer as ContainerLayer;
+  Layer? container = RendererBinding.instance!.renderView.debugLayer;
+  while (container is ContainerLayer) {
     layers.add(container.runtimeType);
     expect(container.firstChild, same(container.lastChild));
-    layer = container.firstChild;
+    container = container.firstChild;
   }
   return layers;
 }
diff --git a/packages/flutter/test/widgets/flex_test.dart b/packages/flutter/test/widgets/flex_test.dart
index b6b15e9c8f..a537af60ae 100644
--- a/packages/flutter/test/widgets/flex_test.dart
+++ b/packages/flutter/test/widgets/flex_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -63,18 +61,6 @@ void main() {
     expect(box.size.width, 100.0);
   });
 
-  testWidgets('Can pass null for flex', (WidgetTester tester) async {
-    await tester.pumpWidget(
-      Row(
-        textDirection: TextDirection.ltr,
-        children: const <Widget>[
-          Expanded(flex: null, child: Text('one', textDirection: TextDirection.ltr)),
-          Flexible(flex: null, child: Text('two', textDirection: TextDirection.ltr)),
-        ],
-      ),
-    );
-  });
-
   testWidgets("Doesn't overflow because of floating point accumulated error", (WidgetTester tester) async {
     // both of these cases have failed in the past due to floating point issues
     await tester.pumpWidget(
diff --git a/packages/flutter/test/widgets/flow_test.dart b/packages/flutter/test/widgets/flow_test.dart
index 6c8cca4046..f25f82d570 100644
--- a/packages/flutter/test/widgets/flow_test.dart
+++ b/packages/flutter/test/widgets/flow_test.dart
@@ -2,14 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 class TestFlowDelegate extends FlowDelegate {
-  TestFlowDelegate({this.startOffset}) : super(repaint: startOffset);
+  TestFlowDelegate({required this.startOffset}) : super(repaint: startOffset);
 
   final Animation<double> startOffset;
 
@@ -23,7 +21,7 @@ class TestFlowDelegate extends FlowDelegate {
     double dy = startOffset.value;
     for (int i = 0; i < context.childCount; ++i) {
       context.paintChild(i, transform: Matrix4.translationValues(0.0, dy, 0.0));
-      dy += 0.75 * context.getChildSize(i).height;
+      dy += 0.75 * context.getChildSize(i)!.height;
     }
   }
 
@@ -149,13 +147,13 @@ void main() {
         ],
       ),
     );
-    ContainerLayer layer = RendererBinding.instance.renderView.debugLayer;
+    ContainerLayer? layer = RendererBinding.instance!.renderView.debugLayer;
     while (layer != null && layer is! OpacityLayer)
-      layer = layer.firstChild as ContainerLayer;
+      layer = layer.firstChild as ContainerLayer?;
     expect(layer, isA<OpacityLayer>());
-    final OpacityLayer opacityLayer = layer as OpacityLayer;
-    expect(opacityLayer.alpha, equals(opacity * 255));
-    expect(layer.firstChild, isA<TransformLayer>());
+    final OpacityLayer? opacityLayer = layer as OpacityLayer?;
+    expect(opacityLayer!.alpha, equals(opacity * 255));
+    expect(layer!.firstChild, isA<TransformLayer>());
   });
 
   testWidgets('Flow can set and update clipBehavior', (WidgetTester tester) async {
diff --git a/packages/flutter/test/widgets/focus_manager_test.dart b/packages/flutter/test/widgets/focus_manager_test.dart
index 74dc015ed1..0cc11a00bb 100644
--- a/packages/flutter/test/widgets/focus_manager_test.dart
+++ b/packages/flutter/test/widgets/focus_manager_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
@@ -18,7 +16,7 @@ void main() {
   final GlobalKey widgetKey = GlobalKey();
   Future<BuildContext> setupWidget(WidgetTester tester) async {
     await tester.pumpWidget(Container(key: widgetKey));
-    return widgetKey.currentContext;
+    return widgetKey.currentContext!;
   }
 
   group(FocusNode, () {
@@ -931,7 +929,7 @@ void main() {
     }, variant: TargetPlatformVariant.all());
     testWidgets('Mouse events change initial focus highlight mode on mobile.', (WidgetTester tester) async {
       expect(FocusManager.instance.highlightMode, equals(FocusHighlightMode.touch));
-      RendererBinding.instance.initMouseTracker(); // Clear out the mouse state.
+      RendererBinding.instance!.initMouseTracker(); // Clear out the mouse state.
       final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse, pointer: 0);
       addTearDown(gesture.removePointer);
       await gesture.moveTo(Offset.zero);
@@ -939,7 +937,7 @@ void main() {
     }, variant: TargetPlatformVariant.mobile());
     testWidgets('Mouse events change initial focus highlight mode on desktop.', (WidgetTester tester) async {
       expect(FocusManager.instance.highlightMode, equals(FocusHighlightMode.traditional));
-      RendererBinding.instance.initMouseTracker(); // Clear out the mouse state.
+      RendererBinding.instance!.initMouseTracker(); // Clear out the mouse state.
       final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse, pointer: 0);
       addTearDown(gesture.removePointer);
       await gesture.moveTo(Offset.zero);
@@ -952,7 +950,7 @@ void main() {
     testWidgets('Events change focus highlight mode.', (WidgetTester tester) async {
       await setupWidget(tester);
       int callCount = 0;
-      FocusHighlightMode lastMode;
+      FocusHighlightMode? lastMode;
       void handleModeChange(FocusHighlightMode mode) {
         lastMode = mode;
         callCount++;