[NNBD] More widget tests (#68088)

* NNBD widget test migration

packages/flutter/test/widgets/intrinsic_width_test.dart

@@ -2,15 +2,13 @@
 // 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';
 
 void main() {
   testWidgets('Intrinsic stepWidth, stepHeight', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/25224
-    Widget buildFrame(double stepWidth, double stepHeight) {
+    Widget buildFrame(double? stepWidth, double? stepHeight) {
       return Center(
         child: IntrinsicWidth(
           stepWidth: stepWidth,

packages/flutter/test/widgets/invert_colors_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/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -49,13 +47,13 @@ void main() {
 // and [invertColors] applied for testing the invert colors.
 class InvertColorTestWidget extends LeafRenderObjectWidget {
   const InvertColorTestWidget({
-    this.color,
+    required this.color,
     this.filter,
-    Key key,
+    Key? key,
   }) : super(key: key);
 
   final Color color;
-  final ColorFilter filter;
+  final ColorFilter? filter;
 
   @override
   RenderInvertColorTest createRenderObject(BuildContext context) {
@@ -83,9 +81,9 @@ class RenderInvertColorTest extends RenderProxyBox {
   }
 
 
-  ColorFilter get filter => _filter;
-  ColorFilter _filter;
-  set filter(ColorFilter value) {
+  ColorFilter? get filter => _filter;
+  ColorFilter? _filter;
+  set filter(ColorFilter? value) {
     if (filter == value)
       return;
     _filter = value;

packages/flutter/test/widgets/keep_alive_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
-
 @TestOn('!chrome') // diagnostics use Platform.operatingSystem.
 import 'dart:io' show Platform;
 
@@ -11,7 +9,10 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
 class Leaf extends StatefulWidget {
-  const Leaf({ Key key, this.child }) : super(key: key);
+  const Leaf({
+    Key? key,
+    required this.child,
+  }) : super(key: key);
   final Widget child;
   @override
   _LeafState createState() => _LeafState();
@@ -73,7 +74,7 @@ void main() {
     expect(find.byKey(const GlobalObjectKey<_LeafState>(60)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(61)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(90), skipOffstage: false), findsNothing);
-    const GlobalObjectKey<_LeafState>(60).currentState.setKeepAlive(true);
+    const GlobalObjectKey<_LeafState>(60).currentState!.setKeepAlive(true);
     await tester.drag(find.byType(ListView), const Offset(0.0, 300.0)); // back to top
     await tester.pump();
     expect(find.byKey(const GlobalObjectKey<_LeafState>(3)), findsOneWidget);
@@ -83,7 +84,7 @@ void main() {
     expect(find.byKey(const GlobalObjectKey<_LeafState>(60)), findsNothing);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(61), skipOffstage: false), findsNothing);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(90), skipOffstage: false), findsNothing);
-    const GlobalObjectKey<_LeafState>(60).currentState.setKeepAlive(false);
+    const GlobalObjectKey<_LeafState>(60).currentState!.setKeepAlive(false);
     await tester.pump();
     expect(find.byKey(const GlobalObjectKey<_LeafState>(3)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(30)), findsOneWidget);
@@ -120,7 +121,7 @@ void main() {
     expect(find.byKey(const GlobalObjectKey<_LeafState>(60)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(61)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(90), skipOffstage: false), findsNothing);
-    const GlobalObjectKey<_LeafState>(60).currentState.setKeepAlive(true);
+    const GlobalObjectKey<_LeafState>(60).currentState!.setKeepAlive(true);
     await tester.drag(find.byType(ListView), const Offset(0.0, 300.0)); // back to top
     await tester.pump();
     expect(find.byKey(const GlobalObjectKey<_LeafState>(3)), findsOneWidget);
@@ -130,7 +131,7 @@ void main() {
     expect(find.byKey(const GlobalObjectKey<_LeafState>(60)), findsNothing);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(61), skipOffstage: false), findsNothing);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(90), skipOffstage: false), findsNothing);
-    const GlobalObjectKey<_LeafState>(60).currentState.setKeepAlive(false);
+    const GlobalObjectKey<_LeafState>(60).currentState!.setKeepAlive(false);
     await tester.pump();
     expect(find.byKey(const GlobalObjectKey<_LeafState>(3)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(30)), findsOneWidget);
@@ -169,7 +170,7 @@ void main() {
     expect(find.byKey(const GlobalObjectKey<_LeafState>(60)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(61)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(90), skipOffstage: false), findsNothing);
-    const GlobalObjectKey<_LeafState>(60).currentState.setKeepAlive(true);
+    const GlobalObjectKey<_LeafState>(60).currentState!.setKeepAlive(true);
     await tester.drag(find.byType(GridView), const Offset(0.0, 300.0)); // back to top
     await tester.pump();
     expect(find.byKey(const GlobalObjectKey<_LeafState>(3)), findsOneWidget);
@@ -179,7 +180,7 @@ void main() {
     expect(find.byKey(const GlobalObjectKey<_LeafState>(60)), findsNothing);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(61), skipOffstage: false), findsNothing);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(90), skipOffstage: false), findsNothing);
-    const GlobalObjectKey<_LeafState>(60).currentState.setKeepAlive(false);
+    const GlobalObjectKey<_LeafState>(60).currentState!.setKeepAlive(false);
     await tester.pump();
     expect(find.byKey(const GlobalObjectKey<_LeafState>(3)), findsOneWidget);
     expect(find.byKey(const GlobalObjectKey<_LeafState>(30)), findsOneWidget);
@@ -345,10 +346,10 @@ void main() {
       '                             constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                             size: Size(800.0, 400.0)\n'
     ));
-    const GlobalObjectKey<_LeafState>(0).currentState.setKeepAlive(true);
+    const GlobalObjectKey<_LeafState>(0).currentState!.setKeepAlive(true);
     await tester.drag(find.byType(ListView), const Offset(0.0, -1000.0));
     await tester.pump();
-    const GlobalObjectKey<_LeafState>(3).currentState.setKeepAlive(true);
+    const GlobalObjectKey<_LeafState>(3).currentState!.setKeepAlive(true);
     await tester.drag(find.byType(ListView), const Offset(0.0, -1000.0));
     await tester.pump();
     expect(tester.binding.renderView.toStringDeep(minLevel: DiagnosticLevel.info), equalsIgnoringHashCodes(

packages/flutter/test/widgets/key_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';
 

packages/flutter/test/widgets/layout_builder_and_global_keys_test.dart

@@ -2,16 +2,14 @@
 // 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/src/rendering/sliver.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
 class Wrapper extends StatelessWidget {
   const Wrapper({
-    Key key,
-    this.child,
+    Key? key,
+    required this.child,
   }) : super(key: key);
 
   final Widget child;
@@ -22,8 +20,8 @@ class Wrapper extends StatelessWidget {
 
 class StatefulWrapper extends StatefulWidget {
   const StatefulWrapper({
-    Key key,
-    this.child,
+    Key? key,
+    required this.child,
   }) : super(key: key);
 
   final Widget child;
@@ -54,7 +52,7 @@ void main() {
     );
     await tester.pumpWidget(
       LayoutBuilder(builder: (BuildContext context, BoxConstraints constraints) {
-        key.currentState.trigger();
+        key.currentState!.trigger();
         return StatefulWrapper(key: key, child: Container(height: 100.0));
       }),
     );
@@ -89,7 +87,7 @@ void main() {
           slivers: <Widget>[
             SliverLayoutBuilder(
               builder: (BuildContext context, SliverConstraints constraint) {
-                key.currentState.trigger();
+                key.currentState!.trigger();
                 return SliverToBoxAdapter(
                   child: StatefulWrapper(key: key, child: Container(height: 100.0)),
                 );

packages/flutter/test/widgets/layout_builder_and_parent_data_test.dart

@@ -2,15 +2,13 @@
 // 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';
 
 class SizeChanger extends StatefulWidget {
   const SizeChanger({
-    Key key,
-    this.child,
+    Key? key,
+    required this.child,
   }) : super(key: key);
 
   final Widget child;
@@ -45,8 +43,8 @@ class SizeChangerState extends State<SizeChanger> {
 
 class Wrapper extends StatelessWidget {
   const Wrapper({
-    Key key,
-    this.child,
+    Key? key,
+    required this.child,
   }) : super(key: key);
 
   final Widget child;

packages/flutter/test/widgets/layout_builder_and_state_test.dart

@@ -2,16 +2,14 @@
 // 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';
 import 'test_widgets.dart';
 
 class StatefulWrapper extends StatefulWidget {
   const StatefulWrapper({
-    Key key,
-    this.child,
+    Key? key,
+    required this.child,
   }) : super(key: key);
 
   final Widget child;
@@ -37,8 +35,8 @@ class StatefulWrapperState extends State<StatefulWrapper> {
 
 class Wrapper extends StatelessWidget {
   const Wrapper({
-    Key key,
-    this.child,
+    Key? key,
+    required this.child,
   }) : super(key: key);
 
   final Widget child;

packages/flutter/test/widgets/layout_builder_mutations_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/src/rendering/sliver.dart';
 import 'package:flutter/src/widgets/basic.dart';
 import 'package:flutter/src/widgets/framework.dart';
@@ -14,8 +12,8 @@ import 'package:flutter_test/flutter_test.dart';
 
 class Wrapper extends StatelessWidget {
   const Wrapper({
-    Key key,
-    @required this.child,
+    Key? key,
+    required this.child,
   }) : assert(child != null),
        super(key: key);
 

packages/flutter/test/widgets/layout_builder_test.dart

@@ -2,15 +2,13 @@
 // 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';
 
 void main() {
   testWidgets('LayoutBuilder parent size', (WidgetTester tester) async {
-    Size layoutBuilderSize;
+    late Size layoutBuilderSize;
     final Key childKey = UniqueKey();
     final Key parentKey = UniqueKey();
 
@@ -41,8 +39,8 @@ void main() {
   });
 
   testWidgets('SliverLayoutBuilder parent geometry', (WidgetTester tester) async {
-    SliverConstraints parentConstraints1;
-    SliverConstraints parentConstraints2;
+    late SliverConstraints parentConstraints1;
+    late SliverConstraints parentConstraints2;
     final Key childKey1 = UniqueKey();
     final Key parentKey1 = UniqueKey();
     final Key childKey2 = UniqueKey();
@@ -81,8 +79,8 @@ void main() {
     final RenderSliver parentSliver2 = tester.renderObject(find.byKey(parentKey2));
 
     // scrollExtent == top + bottom.
-    expect(parentSliver1.geometry.scrollExtent, 2 + 4);
-    expect(parentSliver2.geometry.scrollExtent, 7 + 13);
+    expect(parentSliver1.geometry!.scrollExtent, 2 + 4);
+    expect(parentSliver2.geometry!.scrollExtent, 7 + 13);
 
     final RenderSliver childSliver1 = tester.renderObject(find.byKey(childKey1));
     final RenderSliver childSliver2 = tester.renderObject(find.byKey(childKey2));
@@ -91,8 +89,8 @@ void main() {
   });
 
   testWidgets('LayoutBuilder stateful child', (WidgetTester tester) async {
-    Size layoutBuilderSize;
-    StateSetter setState;
+    late Size layoutBuilderSize;
+    late StateSetter setState;
     final Key childKey = UniqueKey();
     final Key parentKey = UniqueKey();
     double childWidth = 10.0;
@@ -137,7 +135,7 @@ void main() {
   });
 
   testWidgets('SliverLayoutBuilder stateful descendants', (WidgetTester tester) async {
-    StateSetter setState;
+    late StateSetter setState;
     double childWidth = 10.0;
     double childHeight = 20.0;
     final Key parentKey = UniqueKey();
@@ -174,8 +172,8 @@ void main() {
     RenderSliver parentSliver = tester.renderObject(find.byKey(parentKey));
     expect(childBox.size.width, 800);
     expect(childBox.size.height, childHeight);
-    expect(parentSliver.geometry.scrollExtent, childHeight);
-    expect(parentSliver.geometry.paintExtent, childHeight);
+    expect(parentSliver.geometry!.scrollExtent, childHeight);
+    expect(parentSliver.geometry!.paintExtent, childHeight);
 
     setState(() {
       childWidth = 100.0;
@@ -187,8 +185,8 @@ void main() {
     parentSliver = tester.renderObject(find.byKey(parentKey));
     expect(childBox.size.width, 800);
     expect(childBox.size.height, childHeight);
-    expect(parentSliver.geometry.scrollExtent, childHeight);
-    expect(parentSliver.geometry.paintExtent, childHeight);
+    expect(parentSliver.geometry!.scrollExtent, childHeight);
+    expect(parentSliver.geometry!.paintExtent, childHeight);
 
     // Make child wider and higher than the viewport.
     setState(() {
@@ -201,13 +199,13 @@ void main() {
     parentSliver = tester.renderObject(find.byKey(parentKey));
     expect(childBox.size.width, 800);
     expect(childBox.size.height, childHeight);
-    expect(parentSliver.geometry.scrollExtent, childHeight);
-    expect(parentSliver.geometry.paintExtent, 600);
+    expect(parentSliver.geometry!.scrollExtent, childHeight);
+    expect(parentSliver.geometry!.paintExtent, 600);
   });
 
   testWidgets('LayoutBuilder stateful parent', (WidgetTester tester) async {
-    Size layoutBuilderSize;
-    StateSetter setState;
+    late Size layoutBuilderSize;
+    late StateSetter setState;
     final Key childKey = UniqueKey();
     double childWidth = 10.0;
     double childHeight = 20.0;
@@ -362,8 +360,8 @@ void main() {
   });
 
   testWidgets('nested SliverLayoutBuilder', (WidgetTester tester) async {
-    SliverConstraints parentConstraints1;
-    SliverConstraints parentConstraints2;
+    late SliverConstraints parentConstraints1;
+    late SliverConstraints parentConstraints2;
     final Key childKey = UniqueKey();
     final Key parentKey1 = UniqueKey();
     final Key parentKey2 = UniqueKey();
@@ -399,7 +397,7 @@ void main() {
     final RenderSliver parentSliver1 = tester.renderObject(find.byKey(parentKey1));
     final RenderSliver parentSliver2 = tester.renderObject(find.byKey(parentKey2));
     // scrollExtent == top + bottom.
-    expect(parentSliver1.geometry.scrollExtent, 2 + 4);
+    expect(parentSliver1.geometry!.scrollExtent, 2 + 4);
 
     final RenderSliver childSliver = tester.renderObject(find.byKey(childKey));
     expect(childSliver.geometry, parentSliver1.geometry);

packages/flutter/test/widgets/linked_scroll_view_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
-
 // This file contains a wacky demonstration of creating a custom ScrollPosition
 // setup. It's testing that we don't regress the factoring of the
 // ScrollPosition/ScrollActivity logic into a state where you can no longer
@@ -21,18 +19,18 @@ import 'package:flutter_test/flutter_test.dart';
 class LinkedScrollController extends ScrollController {
   LinkedScrollController({ this.before, this.after });
 
-  LinkedScrollController before;
-  LinkedScrollController after;
+  LinkedScrollController? before;
+  LinkedScrollController? after;
 
-  ScrollController _parent;
+  ScrollController? _parent;
 
-  void setParent(ScrollController newParent) {
+  void setParent(ScrollController? newParent) {
     if (_parent != null) {
-      positions.forEach(_parent.detach);
+      positions.forEach(_parent!.detach);
     }
     _parent = newParent;
     if (_parent != null) {
-      positions.forEach(_parent.attach);
+      positions.forEach(_parent!.attach);
     }
   }
 
@@ -54,13 +52,13 @@ class LinkedScrollController extends ScrollController {
   @override
   void dispose() {
     if (_parent != null) {
-      positions.forEach(_parent.detach);
+      positions.forEach(_parent!.detach);
     }
     super.dispose();
   }
 
   @override
-  LinkedScrollPosition createScrollPosition(ScrollPhysics physics, ScrollContext context, ScrollPosition oldPosition) {
+  LinkedScrollPosition createScrollPosition(ScrollPhysics physics, ScrollContext context, ScrollPosition? oldPosition) {
     return LinkedScrollPosition(
       this,
       physics: physics,
@@ -70,18 +68,18 @@ class LinkedScrollController extends ScrollController {
     );
   }
 
-  bool get canLinkWithBefore => before != null && before.hasClients;
+  bool get canLinkWithBefore => before != null && before!.hasClients;
 
-  bool get canLinkWithAfter => after != null && after.hasClients;
+  bool get canLinkWithAfter => after != null && after!.hasClients;
 
   Iterable<LinkedScrollActivity> linkWithBefore(LinkedScrollPosition driver) {
     assert(canLinkWithBefore);
-    return before.link(driver);
+    return before!.link(driver);
   }
 
   Iterable<LinkedScrollActivity> linkWithAfter(LinkedScrollPosition driver) {
     assert(canLinkWithAfter);
-    return after.link(driver);
+    return after!.link(driver);
   }
 
   Iterable<LinkedScrollActivity> link(LinkedScrollPosition driver) sync* {
@@ -109,10 +107,10 @@ class LinkedScrollController extends ScrollController {
 class LinkedScrollPosition extends ScrollPositionWithSingleContext {
   LinkedScrollPosition(
     this.owner, {
-    ScrollPhysics physics,
-    ScrollContext context,
-    double initialPixels,
-    ScrollPosition oldPosition,
+    required ScrollPhysics physics,
+    required ScrollContext context,
+    required double initialPixels,
+    ScrollPosition? oldPosition,
   }) : assert(owner != null),
        super(
          physics: physics,
@@ -123,22 +121,22 @@ class LinkedScrollPosition extends ScrollPositionWithSingleContext {
 
   final LinkedScrollController owner;
 
-  Set<LinkedScrollActivity> _beforeActivities;
-  Set<LinkedScrollActivity> _afterActivities;
+  Set<LinkedScrollActivity>? _beforeActivities;
+  Set<LinkedScrollActivity>? _afterActivities;
 
   @override
-  void beginActivity(ScrollActivity newActivity) {
+  void beginActivity(ScrollActivity? newActivity) {
     if (newActivity == null)
       return;
     if (_beforeActivities != null) {
-      for (final LinkedScrollActivity activity in _beforeActivities)
+      for (final LinkedScrollActivity activity in _beforeActivities!)
         activity.unlink(this);
-      _beforeActivities.clear();
+      _beforeActivities!.clear();
     }
     if (_afterActivities != null) {
-      for (final LinkedScrollActivity activity in _afterActivities)
+      for (final LinkedScrollActivity activity in _afterActivities!)
         activity.unlink(this);
-      _afterActivities.clear();
+      _afterActivities!.clear();
     }
     super.beginActivity(newActivity);
   }
@@ -155,8 +153,8 @@ class LinkedScrollPosition extends ScrollPositionWithSingleContext {
     if (owner.canLinkWithBefore && (value < minScrollExtent)) {
       final double delta = value - minScrollExtent;
       _beforeActivities ??= HashSet<LinkedScrollActivity>();
-      _beforeActivities.addAll(owner.linkWithBefore(this));
-      for (final LinkedScrollActivity activity in _beforeActivities)
+      _beforeActivities!.addAll(owner.linkWithBefore(this));
+      for (final LinkedScrollActivity activity in _beforeActivities!)
         beforeOverscroll = math.min(activity.moveBy(delta), beforeOverscroll);
       assert(beforeOverscroll <= 0.0);
     }
@@ -165,8 +163,8 @@ class LinkedScrollPosition extends ScrollPositionWithSingleContext {
     if (owner.canLinkWithAfter && (value > maxScrollExtent)) {
       final double delta = value - maxScrollExtent;
       _afterActivities ??= HashSet<LinkedScrollActivity>();
-      _afterActivities.addAll(owner.linkWithAfter(this));
-      for (final LinkedScrollActivity activity in _afterActivities)
+      _afterActivities!.addAll(owner.linkWithAfter(this));
+      for (final LinkedScrollActivity activity in _afterActivities!)
         afterOverscroll = math.max(activity.moveBy(delta), afterOverscroll);
       assert(afterOverscroll >= 0.0);
     }
@@ -176,7 +174,7 @@ class LinkedScrollPosition extends ScrollPositionWithSingleContext {
     final double localOverscroll = setPixels(value.clamp(
       owner.canLinkWithBefore ? minScrollExtent : -double.infinity,
       owner.canLinkWithAfter ? maxScrollExtent : double.infinity,
-    ) as double);
+    ));
 
     assert(localOverscroll == 0.0 || (beforeOverscroll == 0.0 && afterOverscroll == 0.0));
   }
@@ -188,16 +186,16 @@ class LinkedScrollPosition extends ScrollPositionWithSingleContext {
   LinkedScrollActivity link(LinkedScrollPosition driver) {
     if (this.activity is! LinkedScrollActivity)
       beginActivity(LinkedScrollActivity(this));
-    final LinkedScrollActivity activity = this.activity as LinkedScrollActivity;
-    activity.link(driver);
+    final LinkedScrollActivity? activity = this.activity as LinkedScrollActivity?;
+    activity!.link(driver);
     return activity;
   }
 
   void unlink(LinkedScrollActivity activity) {
     if (_beforeActivities != null)
-      _beforeActivities.remove(activity);
+      _beforeActivities!.remove(activity);
     if (_afterActivities != null)
-      _afterActivities.remove(activity);
+      _afterActivities!.remove(activity);
   }
 
   @override
@@ -224,7 +222,7 @@ class LinkedScrollActivity extends ScrollActivity {
   void unlink(LinkedScrollPosition driver) {
     drivers.remove(driver);
     if (drivers.isEmpty)
-      delegate?.goIdle();
+      delegate.goIdle();
   }
 
   @override
@@ -240,13 +238,16 @@ class LinkedScrollActivity extends ScrollActivity {
 
   double moveBy(double delta) {
     assert(drivers.isNotEmpty);
-    ScrollDirection commonDirection;
+    ScrollDirection? commonDirection;
     for (final LinkedScrollPosition driver in drivers) {
       commonDirection ??= driver.userScrollDirection;
       if (driver.userScrollDirection != commonDirection)
         commonDirection = ScrollDirection.idle;
     }
-    delegate._userMoved(commonDirection);
+
+    if (commonDirection != null) {
+      delegate._userMoved(commonDirection);
+    }
     return delegate.setPixels(delegate.pixels + delta);
   }
 
@@ -259,14 +260,14 @@ class LinkedScrollActivity extends ScrollActivity {
 }
 
 class Test extends StatefulWidget {
-  const Test({ Key key }) : super(key: key);
+  const Test({ Key? key }) : super(key: key);
   @override
   _TestState createState() => _TestState();
 }
 
 class _TestState extends State<Test> {
-  LinkedScrollController _beforeController;
-  LinkedScrollController _afterController;
+  late LinkedScrollController _beforeController;
+  late LinkedScrollController _afterController;
 
   @override
   void initState() {

packages/flutter/test/widgets/list_body_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/src/foundation/assertions.dart';
 import 'package:flutter/src/painting/basic_types.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -112,7 +110,7 @@ void main() {
   });
 
   testWidgets('Limited space along main axis error', (WidgetTester tester) async {
-    final FlutterExceptionHandler oldHandler = FlutterError.onError;
+    final FlutterExceptionHandler oldHandler = FlutterError.onError!;
     final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
     FlutterError.onError = (FlutterErrorDetails error) => errors.add(error);
     try {
@@ -145,7 +143,7 @@ void main() {
   });
 
   testWidgets('Nested ListBody unbounded cross axis error', (WidgetTester tester) async {
-    final FlutterExceptionHandler oldHandler = FlutterError.onError;
+    final FlutterExceptionHandler oldHandler = FlutterError.onError!;
     final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
     FlutterError.onError = (FlutterErrorDetails error) => errors.add(error);
     try {

packages/flutter/test/widgets/list_view_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';
 
@@ -264,7 +262,7 @@ void main() {
   });
 
   testWidgets('ListView.separated', (WidgetTester tester) async {
-    Widget buildFrame({ int itemCount }) {
+    Widget buildFrame({ required int itemCount }) {
       return Directionality(
         textDirection: TextDirection.ltr,
         child: ListView.separated(
@@ -309,7 +307,7 @@ void main() {
 
 
   testWidgets('ListView.separated uses correct semanticChildCount', (WidgetTester tester) async {
-    Widget buildFrame({int itemCount}) {
+    Widget buildFrame({ required int itemCount}) {
       return Directionality(
         textDirection: TextDirection.ltr,
         child: ListView.separated(

packages/flutter/test/widgets/list_view_correction_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/material.dart';
 

packages/flutter/test/widgets/list_view_fling_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/material.dart';
 

packages/flutter/test/widgets/list_view_horizontal_test.dart

@@ -2,15 +2,13 @@
 // 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';
 
 const List<int> items = <int>[0, 1, 2, 3, 4, 5];
 
-Widget buildFrame({ bool reverse = false, @required TextDirection textDirection }) {
+Widget buildFrame({ bool reverse = false, required TextDirection textDirection }) {
   return Directionality(
     textDirection: textDirection,
     child: Center(

packages/flutter/test/widgets/list_view_misc_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';
@@ -218,7 +216,7 @@ void main() {
     await tester.pumpWidget(buildFrame());
     expect(find.text('top'), findsOneWidget);
 
-    final ScrollPosition position = Scrollable.of(tester.element(find.text('middle'))).position;
+    final ScrollPosition position = Scrollable.of(tester.element(find.text('middle')))!.position;
     expect(position.viewportDimension, 600.0);
     expect(position.pixels, 0.0);
 

packages/flutter/test/widgets/list_view_relayout_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/material.dart';
 import 'package:flutter/rendering.dart';
@@ -80,7 +78,7 @@ void main() {
     );
 
     final RenderSliverList list = tester.renderObject(find.byType(SliverList));
-    expect(list.geometry.scrollExtent, equals(100.0));
+    expect(list.geometry!.scrollExtent, equals(100.0));
 
     await tester.pumpWidget(
       Directionality(
@@ -93,7 +91,7 @@ void main() {
         ),
       ),
     );
-    expect(list.geometry.scrollExtent, equals(300.0));
+    expect(list.geometry!.scrollExtent, equals(300.0));
 
     await tester.pumpWidget(
       Directionality(
@@ -103,7 +101,7 @@ void main() {
         ),
       ),
     );
-    expect(list.geometry.scrollExtent, equals(0.0));
+    expect(list.geometry!.scrollExtent, equals(0.0));
   });
 
   testWidgets('Overflowing ListView should relayout for missing children', (WidgetTester tester) async {

packages/flutter/test/widgets/list_view_semantics_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';
 

packages/flutter/test/widgets/list_view_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/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
@@ -24,7 +22,7 @@ class TestSliverChildListDelegate extends SliverChildListDelegate {
 }
 
 class Alive extends StatefulWidget {
-  const Alive(this.alive, this.index, { Key key }) : super(key: key);
+  const Alive(this.alive, this.index, { Key? key }) : super(key: key);
   final bool alive;
   final int index;
 
@@ -371,7 +369,7 @@ void main() {
   });
 
   testWidgets('ListView automatically pad MediaQuery on axis', (WidgetTester tester) async {
-    EdgeInsets innerMediaQueryPadding;
+    EdgeInsets? innerMediaQueryPadding;
 
     await tester.pumpWidget(
       Directionality(
@@ -384,7 +382,7 @@ void main() {
             children: <Widget>[
               const Text('top', textDirection: TextDirection.ltr),
               Builder(builder: (BuildContext context) {
-                innerMediaQueryPadding = MediaQuery.of(context).padding;
+                innerMediaQueryPadding = MediaQuery.of(context)!.padding;
                 return Container();
               }),
             ],
@@ -545,7 +543,7 @@ void main() {
     // Regression test for https://github.com/flutter/flutter/issues/43380.
     final ScrollController controller = ScrollController();
 
-    Widget buildListView({@required Axis scrollDirection}) {
+    Widget buildListView({ required Axis scrollDirection }) {
       assert(scrollDirection != null);
       return Directionality(
         textDirection: TextDirection.ltr,

packages/flutter/test/widgets/list_view_vertical_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';
 

packages/flutter/test/widgets/list_view_viewporting_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/material.dart';
 import 'package:flutter/rendering.dart';
@@ -182,7 +180,7 @@ void main() {
 
   testWidgets('ListView reinvoke builders', (WidgetTester tester) async {
     final List<int> callbackTracker = <int>[];
-    final List<String> text = <String>[];
+    final List<String?> text = <String?>[];
 
     final IndexedWidgetBuilder itemBuilder = (BuildContext context, int index) {
       callbackTracker.add(index);
@@ -232,7 +230,7 @@ void main() {
   });
 
   testWidgets('ListView reinvoke builders', (WidgetTester tester) async {
-    StateSetter setState;
+    late StateSetter setState;
     ThemeData themeData = ThemeData.light();
 
     final IndexedWidgetBuilder itemBuilder = (BuildContext context, int index) {
@@ -240,7 +238,7 @@ void main() {
         key: ValueKey<int>(index),
         width: 500.0, // this should be ignored
         height: 220.0,
-        color: Theme.of(context).primaryColor,
+        color: Theme.of(context)!.primaryColor,
         child: Text('$index', textDirection: TextDirection.ltr),
       );
     };
@@ -319,10 +317,10 @@ void main() {
 
     final RenderSliverList list = tester.renderObject(find.byType(SliverList));
 
-    expect(list.indexOf(list.firstChild), equals(0));
-    expect(list.indexOf(list.lastChild), equals(2));
-    expect(list.childScrollOffset(list.firstChild), equals(0.0));
-    expect(list.geometry.scrollExtent, equals(300.0));
+    expect(list.indexOf(list.firstChild!), equals(0));
+    expect(list.indexOf(list.lastChild!), equals(2));
+    expect(list.childScrollOffset(list.firstChild!), equals(0.0));
+    expect(list.geometry!.scrollExtent, equals(300.0));
 
     expect(list, hasAGoodToStringDeep);
     expect(

packages/flutter/test/widgets/list_view_with_inherited_test.dart

@@ -2,22 +2,30 @@
 // 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';
 
-List<int> items = <int>[0, 1, 2, 3, 4, 5];
+List<String> items = <String>[
+  'one',
+  'two',
+  'three',
+  'four',
+  'five',
+];
 
 Widget buildCard(BuildContext context, int index) {
-  if (index >= items.length)
-    return null;
+  // We still want to populate the list with items beyond the list
+  // provided.
+  if (index >= items.length) {
+    return const SizedBox(height: 100);
+  }
+
   return Container(
-    key: ValueKey<int>(items[index]),
+    key: ValueKey<String>(items[index]),
     height: 100.0,
     child: DefaultTextStyle(
       style: TextStyle(fontSize: 2.0 + items.length.toDouble()),
-      child: Text('${items[index]}', textDirection: TextDirection.ltr),
+      child: Text(items[index], textDirection: TextDirection.ltr),
     ),
   );
 }
@@ -34,15 +42,15 @@ Widget buildFrame() {
 void main() {
   testWidgets('ListView is a build function (smoketest)', (WidgetTester tester) async {
     await tester.pumpWidget(buildFrame());
-    expect(find.text('0'), findsOneWidget);
-    expect(find.text('1'), findsOneWidget);
-    expect(find.text('2'), findsOneWidget);
-    expect(find.text('3'), findsOneWidget);
+    expect(find.text('one'), findsOneWidget);
+    expect(find.text('two'), findsOneWidget);
+    expect(find.text('three'), findsOneWidget);
+    expect(find.text('four'), findsOneWidget);
     items.removeAt(2);
     await tester.pumpWidget(buildFrame());
-    expect(find.text('0'), findsOneWidget);
-    expect(find.text('1'), findsOneWidget);
-    expect(find.text('2'), findsNothing);
-    expect(find.text('3'), findsOneWidget);
+    expect(find.text('one'), findsOneWidget);
+    expect(find.text('two'), findsOneWidget);
+    expect(find.text('three'), findsNothing);
+    expect(find.text('four'), findsOneWidget);
   });
 }

packages/flutter/test/widgets/list_wheel_scroll_view_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/foundation.dart';
 import 'package:flutter/rendering.dart';
@@ -65,18 +63,6 @@ void main() {
       }
     });
 
-    testWidgets('ListWheelScrollView needs positive item extent', (WidgetTester tester) async {
-      expect(
-        () {
-          ListWheelScrollView(
-            itemExtent: null,
-            children: <Widget>[Container()],
-          );
-        },
-        throwsAssertionError,
-      );
-    });
-
     testWidgets('ListWheelScrollView can have zero child', (WidgetTester tester) async {
       await tester.pumpWidget(
         Directionality(
@@ -106,17 +92,6 @@ void main() {
     });
 
     testWidgets('ListWheelScrollView needs valid overAndUnderCenterOpacity', (WidgetTester tester) async {
-      expect(
-        () {
-          ListWheelScrollView(
-            overAndUnderCenterOpacity: null,
-            itemExtent: 20.0,
-            children: <Widget>[Container()],
-          );
-        },
-        throwsAssertionError,
-      );
-
       expect(
         () {
           ListWheelScrollView(
@@ -426,7 +401,7 @@ void main() {
         ),
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Text)).parent.parent as RenderListWheelViewport;
+      final RenderListWheelViewport viewport = tester.renderObject(find.byType(ListWheelViewport)) as RenderListWheelViewport;
 
       // Item 0 is in the middle. There are 3 children visible after it, so the
       // value of childCount should be 4.
@@ -463,7 +438,7 @@ void main() {
         ),
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Text)).parent.parent as RenderListWheelViewport;
+      final RenderListWheelViewport viewport = tester.renderObject(find.byType(ListWheelViewport)) as RenderListWheelViewport;
 
       // The screen is vertically 600px. Since the middle item is centered,
       // half of the first and last items are visible, making 7 children visible.
@@ -654,7 +629,7 @@ void main() {
         ),
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent.parent as RenderListWheelViewport;
+      final RenderListWheelViewport viewport = tester.renderObject(find.byType(ListWheelViewport)) as RenderListWheelViewport;
       expect(viewport, paints..transform(
         matrix4: equals(<dynamic>[
           1.0, 0.0, 0.0, 0.0,
@@ -711,7 +686,7 @@ void main() {
         ),
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent.parent as RenderListWheelViewport;
+      final RenderListWheelViewport viewport = tester.renderObject(find.byType(ListWheelViewport)) as RenderListWheelViewport;
       expect(viewport, paints..transform(
         matrix4: equals(<dynamic>[
           1.0, 0.0, 0.0, 0.0,
@@ -832,7 +807,7 @@ void main() {
         ),
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent.parent as RenderListWheelViewport;
+      final RenderListWheelViewport viewport = tester.renderObject(find.byType(ListWheelViewport)) as RenderListWheelViewport;
       expect(viewport, paints
         ..transform(
           matrix4: equals(<dynamic>[
@@ -1295,7 +1270,7 @@ void main() {
 
   testWidgets('ListWheelScrollView getOffsetToReveal', (WidgetTester tester) async {
     List<Widget> outerChildren;
-    final List<Widget> innerChildren = List<Widget>(10);
+    final List<Widget> innerChildren = List<Widget>.generate(10, (int index) => Container());
 
     await tester.pumpWidget(
       Directionality(
@@ -1365,7 +1340,7 @@ void main() {
 
   testWidgets('ListWheelScrollView showOnScreen', (WidgetTester tester) async {
     List<Widget> outerChildren;
-    final List<Widget> innerChildren = List<Widget>(10);
+    final List<Widget> innerChildren = List<Widget>.generate(10, (int index) => Container());
     ScrollController controller;
 
     await tester.pumpWidget(

packages/flutter/test/widgets/listener_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/material.dart';

packages/flutter/test/widgets/listview_end_append_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';
 import 'package:flutter/gestures.dart' show DragStartBehavior;

packages/flutter/test/widgets/localizations_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:async';
 
 import 'package:flutter_test/flutter_test.dart';
@@ -22,11 +20,11 @@ void main() {
       ],
       child: const Text('loaded')
     ));
-    final dynamic state = tester.state(find.byType(Localizations));
-    expect(state.locale, isNull);
+    final dynamic state = tester.state(find.byType(Localizations)); // ignore: unnecessary_nullable_for_final_variable_declarations
+    expect(state!.locale, isNull);
     expect(find.text('loaded'), findsNothing);
 
-    Locale locale;
+    late Locale locale;
     binding.onAllowFrame = () {
       locale = state.locale as Locale;
     };
@@ -53,12 +51,11 @@ class FakeLocalizationsDelegate extends LocalizationsDelegate<String> {
 
 class TestAutomatedTestWidgetsFlutterBinding extends AutomatedTestWidgetsFlutterBinding {
 
-  VoidCallback onAllowFrame;
+  VoidCallback? onAllowFrame;
 
   @override
   void allowFirstFrame() {
-    if (onAllowFrame != null)
-      onAllowFrame();
+    onAllowFrame?.call();
     super.allowFirstFrame();
   }
 }

packages/flutter/test/widgets/modal_barrier_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/foundation.dart';
 import 'package:flutter/material.dart';
@@ -15,10 +13,10 @@ import 'package:flutter/gestures.dart' show kSecondaryButton, PointerDeviceKind;
 import 'semantics_tester.dart';
 
 void main() {
-  bool tapped;
-  bool hovered;
-  Widget tapTarget;
-  Widget hoverTarget;
+  late bool tapped;
+  late bool hovered;
+  late Widget tapTarget;
+  late Widget hoverTarget;
 
   setUp(() {
     tapped = false;
@@ -418,12 +416,12 @@ void main() {
 
     await tester.pump();
 
-    expect(RendererBinding.instance.mouseTracker.debugDeviceActiveCursor(1), SystemMouseCursors.basic);
+    expect(RendererBinding.instance!.mouseTracker.debugDeviceActiveCursor(1), SystemMouseCursors.basic);
   });
 }
 
 class FirstWidget extends StatelessWidget {
-  const FirstWidget({ Key key }) : super(key: key);
+  const FirstWidget({ Key? key }) : super(key: key);
   @override
   Widget build(BuildContext context) {
     return GestureDetector(
@@ -438,7 +436,7 @@ class FirstWidget extends StatelessWidget {
 }
 
 class SecondWidget extends StatelessWidget {
-  const SecondWidget({ Key key }) : super(key: key);
+  const SecondWidget({ Key? key }) : super(key: key);
   @override
   Widget build(BuildContext context) {
     return const ModalBarrier(
@@ -449,7 +447,7 @@ class SecondWidget extends StatelessWidget {
 }
 
 class SecondWidgetWithCompetence extends StatelessWidget {
-  const SecondWidgetWithCompetence({ Key key }) : super(key: key);
+  const SecondWidgetWithCompetence({ Key? key }) : super(key: key);
   @override
   Widget build(BuildContext context) {
     return Stack(