add key to constructors of public widgets (#47884)

* add key to constructors of public widgets

* fix a test

* fix a test

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

@@ -265,12 +265,14 @@ class CupertinoActionSheetAction extends StatelessWidget {
   ///
   /// The [child] and [onPressed] arguments must not be null.
   const CupertinoActionSheetAction({
+    Key key,
     @required this.onPressed,
     this.isDefaultAction = false,
     this.isDestructiveAction = false,
     @required this.child,
   }) : assert(child != null),
-       assert(onPressed != null);
+       assert(onPressed != null),
+       super(key: key);
 
   /// The callback that is called when the button is tapped.
   ///

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

@@ -230,6 +230,7 @@ class CupertinoDatePicker extends StatefulWidget {
   ///
   /// [use24hFormat] decides whether 24 hour format is used. Defaults to false.
   CupertinoDatePicker({
+    Key key,
     this.mode = CupertinoDatePickerMode.dateAndTime,
     @required this.onDateTimeChanged,
     DateTime initialDateTime,
@@ -247,7 +248,8 @@ class CupertinoDatePicker extends StatefulWidget {
        assert(
          minuteInterval > 0 && 60 % minuteInterval == 0,
          'minute interval is not a positive integer factor of 60',
-       ) {
+       ),
+       super(key: key) {
     assert(this.initialDateTime != null);
     assert(
       mode != CupertinoDatePickerMode.dateAndTime || minimumDate == null || !this.initialDateTime.isBefore(minimumDate),

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

@@ -1270,11 +1270,13 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
   ///
   /// The [color] parameter must not be null.
   const CupertinoNavigationBarBackButton({
+    Key key,
     this.color,
     this.previousPageTitle,
     this.onPressed,
   }) : _backChevron = null,
-       _backLabel = null;
+       _backLabel = null,
+       super(key: key);
 
   // Allow the back chevron and label to be separately created (and keyed)
   // because they animate separately during page transitions.

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

@@ -200,7 +200,7 @@ class _CupertinoPageScaffoldState extends State<CupertinoPageScaffold> {
 ///
 /// Used by [CupertinoPageScaffold] to either shift away fully obstructed content
 /// or provide a padding guide to partially obstructed content.
-abstract class ObstructingPreferredSizeWidget extends PreferredSizeWidget {
+abstract class ObstructingPreferredSizeWidget implements PreferredSizeWidget {
   /// If true, this widget fully obstructs widgets behind it by the specified
   /// size.
   ///

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

@@ -58,6 +58,7 @@ class ReorderableListView extends StatefulWidget {
 
   /// Creates a reorderable list.
   ReorderableListView({
+    Key key,
     this.header,
     @required this.children,
     @required this.onReorder,
@@ -70,7 +71,8 @@ class ReorderableListView extends StatefulWidget {
        assert(
          children.every((Widget w) => w.key != null),
          'All children of this widget must have a key.',
-       );
+       ),
+       super(key: key);
 
   /// A non-reorderable header widget to show before the list.
   ///

packages/flutter/lib/src/widgets/platform_view.dart

@@ -922,12 +922,14 @@ class PlatformViewSurface extends LeafRenderObjectWidget {
   ///
   /// The [controller] must not be null.
   const PlatformViewSurface({
+    Key key,
     @required this.controller,
     @required this.hitTestBehavior,
     @required this.gestureRecognizers,
   }) : assert(controller != null),
        assert(hitTestBehavior != null),
-       assert(gestureRecognizers != null);
+       assert(gestureRecognizers != null),
+       super(key: key);
 
   /// The controller for the platform view integrated by this [PlatformViewSurface].
   ///

packages/flutter/lib/src/widgets/text.dart

@@ -58,13 +58,14 @@ class DefaultTextStyle extends InheritedTheme {
   ///
   /// This constructor creates a [DefaultTextStyle] that lacks a [child], which
   /// means the constructed value cannot be incorporated into the tree.
-  const DefaultTextStyle.fallback()
+  const DefaultTextStyle.fallback({ Key key })
     : style = const TextStyle(),
       textAlign = null,
       softWrap = true,
       maxLines = null,
       overflow = TextOverflow.clip,
-      textWidthBasis = TextWidthBasis.parent;
+      textWidthBasis = TextWidthBasis.parent,
+      super(key: key, child: null);
 
   /// Creates a default text style that overrides the text styles in scope at
   /// this point in the widget tree.

packages/flutter/lib/src/widgets/value_listenable_builder.dart

@@ -115,11 +115,13 @@ class ValueListenableBuilder<T> extends StatefulWidget {
   /// The [child] is optional but is good practice to use if part of the widget
   /// subtree does not depend on the value of the [valueListenable].
   const ValueListenableBuilder({
+    Key key,
     @required this.valueListenable,
     @required this.builder,
     this.child,
   }) : assert(valueListenable != null),
-       assert(builder != null);
+       assert(builder != null),
+       super(key: key);
 
   /// The [ValueListenable] whose value you depend on in order to build.
   ///

packages/flutter/test/material/bottom_app_bar_test.dart

@@ -419,7 +419,7 @@ class ClipCachePainter extends CustomPainter {
 }
 
 class ShapeListener extends StatefulWidget {
-  const ShapeListener(this.child);
+  const ShapeListener(this.child, { Key key }) : super(key: key);
 
   final Widget child;
 

packages/flutter/test/material/dropdown_form_field_test.dart

@@ -107,10 +107,17 @@ class _TestAppState extends State<TestApp> {
 }
 
 class TestApp extends StatefulWidget {
-  const TestApp({ this.textDirection, this.child, this.mediaSize });
+  const TestApp({
+    Key key,
+    this.textDirection,
+    this.child,
+    this.mediaSize,
+  }) : super(key: key);
+
   final TextDirection textDirection;
   final Widget child;
   final Size mediaSize;
+
   @override
   _TestAppState createState() => _TestAppState();
 }

packages/flutter/test/material/dropdown_test.dart

@@ -93,10 +93,17 @@ Widget buildFrame({
 }
 
 class TestApp extends StatefulWidget {
-  const TestApp({ this.textDirection, this.child, this.mediaSize });
+  const TestApp({
+    Key key,
+    this.textDirection,
+    this.child,
+    this.mediaSize,
+  }) : super(key: key);
+
   final TextDirection textDirection;
   final Widget child;
   final Size mediaSize;
+
   @override
   _TestAppState createState() => _TestAppState();
 }

packages/flutter/test/material/expansion_panel_test.dart

@@ -55,7 +55,7 @@ class _SimpleExpansionPanelListTestWidgetState extends State<SimpleExpansionPane
 }
 
 class ExpansionPanelListSemanticsTest extends StatefulWidget {
-  const ExpansionPanelListSemanticsTest({this.headerKey});
+  const ExpansionPanelListSemanticsTest({ Key key, this.headerKey }) : super(key: key);
 
   final Key headerKey;
 

packages/flutter/test/material/feedback_test.dart

@@ -201,11 +201,11 @@ void main () {
 }
 
 class TestWidget extends StatelessWidget {
-
   const TestWidget({
+    Key key,
     this.tapHandler = nullHandler,
     this.longPressHandler = nullHandler,
-  });
+  }) : super(key: key);
 
   final HandlerCreator tapHandler;
   final HandlerCreator longPressHandler;

packages/flutter/test/material/material_test.dart

@@ -11,6 +11,7 @@ import '../rendering/mock_canvas.dart';
 import '../widgets/test_border.dart' show TestBorder;
 
 class NotifyMaterial extends StatelessWidget {
+  const NotifyMaterial({ Key key }) : super(key: key);
   @override
   Widget build(BuildContext context) {
     LayoutChangedNotification().dispatch(context);
@@ -101,7 +102,7 @@ void main() {
 
   testWidgets('LayoutChangedNotification test', (WidgetTester tester) async {
     await tester.pumpWidget(
-      Material(
+      const Material(
         child: NotifyMaterial(),
       ),
     );

packages/flutter/test/material/popup_menu_test.dart

@@ -1194,9 +1194,15 @@ void main() {
 }
 
 class TestApp extends StatefulWidget {
-  const TestApp({ this.textDirection, this.child });
+  const TestApp({
+    Key key,
+    this.textDirection,
+    this.child,
+  }) : super(key: key);
+
   final TextDirection textDirection;
   final Widget child;
+
   @override
   _TestAppState createState() => _TestAppState();
 }

packages/flutter/test/material/search_test.dart

@@ -644,11 +644,12 @@ void main() {
 
 class TestHomePage extends StatelessWidget {
   const TestHomePage({
+    Key key,
     this.results,
     this.delegate,
     this.passInInitialQuery = false,
     this.initialQuery,
-  });
+  }) : super(key: key);
 
   final List<String> results;
   final SearchDelegate<String> delegate;

packages/flutter/test/material/tabbed_scrollview_warp_test.dart

@@ -24,6 +24,8 @@ class MySliverPersistentHeaderDelegate extends SliverPersistentHeaderDelegate {
 }
 
 class MyHomePage extends StatefulWidget {
+  const MyHomePage({ Key key }) : super(key: key);
+
   @override
   _MyHomePageState createState() => _MyHomePageState();
 }
@@ -73,7 +75,7 @@ class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
 
 void main() {
   testWidgets('Tabbed CustomScrollViews, warp from tab 1 to 3', (WidgetTester tester) async {
-    await tester.pumpWidget(MaterialApp(home: MyHomePage()));
+    await tester.pumpWidget(const MaterialApp(home: MyHomePage()));
 
     // should not crash.
     await tester.tap(find.text('Tab 2'));

packages/flutter/test/material/tabs_test.dart

@@ -127,7 +127,12 @@ Widget buildFrame({
 typedef TabControllerFrameBuilder = Widget Function(BuildContext context, TabController controller);
 
 class TabControllerFrame extends StatefulWidget {
-  const TabControllerFrame({ this.length, this.initialIndex = 0, this.builder });
+  const TabControllerFrame({
+    Key key,
+    this.length,
+    this.initialIndex = 0,
+    this.builder,
+  }) : super(key: key);
 
   final int length;
   final int initialIndex;

packages/flutter/test/material/theme_test.dart

@@ -650,7 +650,7 @@ void main() {
 
 int testBuildCalled;
 class Test extends StatefulWidget {
-  const Test();
+  const Test({ Key key }) : super(key: key);
 
   @override
   _TestState createState() => _TestState();

packages/flutter/test/material/will_pop_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter/material.dart';
 bool willPopValue = false;
 
 class SamplePage extends StatefulWidget {
+  const SamplePage({ Key key }) : super(key: key);
   @override
   SamplePageState createState() => SamplePageState();
 }
@@ -86,7 +87,7 @@ void main() {
                   onPressed: () {
                     showDialog<void>(
                       context: context,
-                      builder: (BuildContext context) => SamplePage(),
+                      builder: (BuildContext context) => const SamplePage(),
                     );
                   },
                 ),

packages/flutter/test/widgets/baseline_test.dart

@@ -94,7 +94,7 @@ void main() {
 }
 
 class BaselineDetector extends LeafRenderObjectWidget {
-  const BaselineDetector(this.callback);
+  const BaselineDetector(this.callback, { Key key }) : super(key: key);
 
   final VoidCallback callback;
 

packages/flutter/test/widgets/build_scope_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter/widgets.dart';
 import 'test_widgets.dart';
 
 class ProbeWidget extends StatefulWidget {
+  const ProbeWidget({ Key key }) : super(key: key);
   @override
   ProbeWidgetState createState() => ProbeWidgetState();
 }
@@ -36,7 +37,7 @@ class ProbeWidgetState extends State<ProbeWidget> {
 }
 
 class BadWidget extends StatelessWidget {
-  const BadWidget(this.parentState);
+  const BadWidget(this.parentState, { Key key }) : super(key: key);
 
   final BadWidgetParentState parentState;
 
@@ -48,6 +49,7 @@ class BadWidget extends StatelessWidget {
 }
 
 class BadWidgetParent extends StatefulWidget {
+  const BadWidgetParent({ Key key }) : super(key: key);
   @override
   BadWidgetParentState createState() => BadWidgetParentState();
 }
@@ -67,6 +69,7 @@ class BadWidgetParentState extends State<BadWidgetParent> {
 }
 
 class BadDisposeWidget extends StatefulWidget {
+  const BadDisposeWidget({ Key key }) : super(key: key);
   @override
   BadDisposeWidgetState createState() => BadDisposeWidgetState();
 }
@@ -133,14 +136,14 @@ void main() {
   testWidgets('Legal times for setState', (WidgetTester tester) async {
     final GlobalKey flipKey = GlobalKey();
     expect(ProbeWidgetState.buildCount, equals(0));
-    await tester.pumpWidget(ProbeWidget());
+    await tester.pumpWidget(const ProbeWidget(key: Key('a')));
     expect(ProbeWidgetState.buildCount, equals(1));
-    await tester.pumpWidget(ProbeWidget());
+    await tester.pumpWidget(const ProbeWidget(key: Key('b')));
     expect(ProbeWidgetState.buildCount, equals(2));
     await tester.pumpWidget(FlipWidget(
       key: flipKey,
       left: Container(),
-      right: ProbeWidget(),
+      right: const ProbeWidget(key: Key('c')),
     ));
     expect(ProbeWidgetState.buildCount, equals(2));
     final FlipWidgetState flipState1 = flipKey.currentState as FlipWidgetState;
@@ -156,13 +159,13 @@ void main() {
   });
 
   testWidgets('Setting parent state during build is forbidden', (WidgetTester tester) async {
-    await tester.pumpWidget(BadWidgetParent());
+    await tester.pumpWidget(const BadWidgetParent());
     expect(tester.takeException(), isFlutterError);
     await tester.pumpWidget(Container());
   });
 
   testWidgets('Setting state during dispose is forbidden', (WidgetTester tester) async {
-    await tester.pumpWidget(BadDisposeWidget());
+    await tester.pumpWidget(const BadDisposeWidget());
     expect(tester.takeException(), isNull);
     await tester.pumpWidget(Container());
     expect(tester.takeException(), isNotNull);

packages/flutter/test/widgets/dismissible_test.dart

@@ -208,7 +208,7 @@ Future<void> rollbackElement(WidgetTester tester, Finder finder, { @required Axi
 }
 
 class Test1215DismissibleWidget extends StatelessWidget {
-  const Test1215DismissibleWidget(this.text);
+  const Test1215DismissibleWidget(this.text, { Key key }) : super(key: key);
 
   final String text;
 

packages/flutter/test/widgets/dispose_ancestor_lookup_test.dart

@@ -8,7 +8,7 @@ import 'package:flutter/widgets.dart';
 typedef TestCallback = void Function(BuildContext context);
 
 class TestWidget extends StatefulWidget {
-  const TestWidget(this.callback);
+  const TestWidget(this.callback, { Key key }) : super(key: key);
 
   final TestCallback callback;
 

packages/flutter/test/widgets/editable_text_test.dart

@@ -4153,11 +4153,13 @@ class MockTextSelectionControls extends Mock implements TextSelectionControls {
 
 class CustomStyleEditableText extends EditableText {
   CustomStyleEditableText({
+    Key key,
     TextEditingController controller,
     Color cursorColor,
     FocusNode focusNode,
     TextStyle style,
   }) : super(
+          key: key,
           controller: controller,
           cursorColor: cursorColor,
           backgroundCursorColor: Colors.grey,
@@ -4180,7 +4182,11 @@ class CustomStyleEditableTextState extends EditableTextState {
 }
 
 class TransformedEditableText extends StatefulWidget {
-  const TransformedEditableText({ this.offset, this.transformButtonKey });
+  const TransformedEditableText({
+    Key key,
+    this.offset,
+    this.transformButtonKey,
+  }) : super(key: key);
 
   final Offset offset;
   final Key transformButtonKey;

packages/flutter/test/widgets/framework_test.dart

@@ -644,7 +644,7 @@ void main() {
   });
 
   testWidgets('Element diagnostics with null child', (WidgetTester tester) async {
-    await tester.pumpWidget(NullChildTest());
+    await tester.pumpWidget(const NullChildTest());
     final NullChildElement test = tester.element<NullChildElement>(find.byType(NullChildTest));
     test.includeChild = true;
     expect(
@@ -713,6 +713,7 @@ void main() {
 }
 
 class NullChildTest extends Widget {
+  const NullChildTest({ Key key }) : super(key: key);
   @override
   Element createElement() => NullChildElement(this);
 }

packages/flutter/test/widgets/global_keys_moving_test.dart

@@ -29,7 +29,7 @@ class StatefulLeafState extends State<StatefulLeaf> {
 }
 
 class KeyedWrapper extends StatelessWidget {
-  const KeyedWrapper(this.key1, this.key2);
+  const KeyedWrapper(this.key1, this.key2, { Key key }) : super(key: key);
 
   final Key key1;
   final GlobalKey key2;

packages/flutter/test/widgets/implicit_animations_test.dart

@@ -248,7 +248,12 @@ Widget wrap({Widget child}) {
 }
 
 class TestAnimatedWidget extends StatefulWidget {
-  const TestAnimatedWidget({this.callback, this.switchKey, this.state});
+  const TestAnimatedWidget({
+    Key key,
+    this.callback,
+    this.switchKey,
+    this.state,
+  }) : super(key: key);
   @required
   final VoidCallback callback;
   @required

packages/flutter/test/widgets/independent_widget_layout_test.dart

@@ -66,9 +66,15 @@ class Trigger {
 }
 
 class TriggerableWidget extends StatefulWidget {
-  const TriggerableWidget({ this.trigger, this.counter });
+  const TriggerableWidget({
+    Key key,
+    this.trigger,
+    this.counter,
+  }) : super(key: key);
+
   final Trigger trigger;
   final Counter counter;
+
   @override
   TriggerableState createState() => TriggerableState();
 }

packages/flutter/test/widgets/inherited_test.dart

@@ -30,7 +30,7 @@ class ValueInherited extends InheritedWidget {
 }
 
 class ExpectFail extends StatefulWidget {
-  const ExpectFail(this.onError);
+  const ExpectFail(this.onError, { Key key }) : super(key: key);
   final VoidCallback onError;
 
   @override

packages/flutter/test/widgets/inherited_theme_test.dart

@@ -12,7 +12,7 @@ class TestRoute extends PageRouteBuilder<void> {
 }
 
 class IconTextBox extends StatelessWidget {
-  const IconTextBox(this.text);
+  const IconTextBox(this.text, { Key key }) : super(key: key);
   final String text;
   @override
   Widget build(BuildContext context) {

packages/flutter/test/widgets/init_state_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter/widgets.dart';
 List<String> ancestors = <String>[];
 
 class TestWidget extends StatefulWidget {
+  const TestWidget({ Key key }) : super(key: key);
   @override
   TestWidgetState createState() => TestWidgetState();
 }
@@ -28,7 +29,7 @@ class TestWidgetState extends State<TestWidget> {
 
 void main() {
   testWidgets('initState() is called when we are in the tree', (WidgetTester tester) async {
-    await tester.pumpWidget(Container(child: TestWidget()));
+    await tester.pumpWidget(Container(child: const TestWidget()));
     expect(ancestors, equals(<String>['Container', 'RenderObjectToWidgetAdapter<RenderBox>']));
   });
 }

packages/flutter/test/widgets/linked_scroll_view_test.dart

@@ -257,6 +257,7 @@ class LinkedScrollActivity extends ScrollActivity {
 }
 
 class Test extends StatefulWidget {
+  const Test({ Key key }) : super(key: key);
   @override
   _TestState createState() => _TestState();
 }
@@ -372,7 +373,7 @@ class _TestState extends State<Test> {
 
 void main() {
   testWidgets('LinkedScrollController - 1', (WidgetTester tester) async {
-    await tester.pumpWidget(Test());
+    await tester.pumpWidget(const Test());
     expect(find.text('Hello A'), findsOneWidget);
     expect(find.text('Hello 1'), findsOneWidget);
     expect(find.text('Hello D'), findsNothing);
@@ -450,7 +451,7 @@ void main() {
     expect(find.text('Hello 4'), findsOneWidget);
   });
   testWidgets('LinkedScrollController - 2', (WidgetTester tester) async {
-    await tester.pumpWidget(Test());
+    await tester.pumpWidget(const Test());
     expect(find.text('Hello A'), findsOneWidget);
     expect(find.text('Hello B'), findsOneWidget);
     expect(find.text('Hello C'), findsNothing);

packages/flutter/test/widgets/list_view_test.dart

@@ -19,7 +19,7 @@ class TestSliverChildListDelegate extends SliverChildListDelegate {
 }
 
 class Alive extends StatefulWidget {
-  const Alive(this.alive, this.index);
+  const Alive(this.alive, this.index, { Key key }) : super(key: key);
   final bool alive;
   final int index;
 

packages/flutter/test/widgets/modal_barrier_test.dart

@@ -165,8 +165,8 @@ void main() {
 
   testWidgets('ModalBarrier pops the Navigator when dismissed by primay tap', (WidgetTester tester) async {
     final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
-      '/': (BuildContext context) => FirstWidget(),
-      '/modal': (BuildContext context) => SecondWidget(),
+      '/': (BuildContext context) => const FirstWidget(),
+      '/modal': (BuildContext context) => const SecondWidget(),
     };
 
     await tester.pumpWidget(MaterialApp(routes: routes));
@@ -195,8 +195,8 @@ void main() {
 
   testWidgets('ModalBarrier pops the Navigator when dismissed by non-primary tap', (WidgetTester tester) async {
     final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
-      '/': (BuildContext context) => FirstWidget(),
-      '/modal': (BuildContext context) => SecondWidget(),
+      '/': (BuildContext context) => const FirstWidget(),
+      '/modal': (BuildContext context) => const SecondWidget(),
     };
 
     await tester.pumpWidget(MaterialApp(routes: routes));
@@ -226,8 +226,8 @@ void main() {
 
   testWidgets('ModalBarrier may pop the Navigator when competing with other gestures', (WidgetTester tester) async {
     final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
-      '/': (BuildContext context) => FirstWidget(),
-      '/modal': (BuildContext context) => SecondWidgetWithCompetence (),
+      '/': (BuildContext context) => const FirstWidget(),
+      '/modal': (BuildContext context) => const SecondWidgetWithCompetence(),
     };
 
     await tester.pumpWidget(MaterialApp(routes: routes));
@@ -252,10 +252,10 @@ void main() {
   testWidgets('ModalBarrier does not pop the Navigator with a WillPopScope that returns false', (WidgetTester tester) async {
     bool willPopCalled = false;
     final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
-      '/': (BuildContext context) => FirstWidget(),
+      '/': (BuildContext context) => const FirstWidget(),
       '/modal': (BuildContext context) => Stack(
         children: <Widget>[
-          SecondWidget(),
+          const SecondWidget(),
           WillPopScope(
             child: const SizedBox(),
             onWillPop: () async {
@@ -292,10 +292,10 @@ void main() {
   testWidgets('ModalBarrier pops the Navigator with a WillPopScope that returns true', (WidgetTester tester) async {
     bool willPopCalled = false;
     final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
-      '/': (BuildContext context) => FirstWidget(),
+      '/': (BuildContext context) => const FirstWidget(),
       '/modal': (BuildContext context) => Stack(
         children: <Widget>[
-          SecondWidget(),
+          const SecondWidget(),
           WillPopScope(
             child: const SizedBox(),
             onWillPop: () async {
@@ -379,6 +379,7 @@ void main() {
 }
 
 class FirstWidget extends StatelessWidget {
+  const FirstWidget({ Key key }) : super(key: key);
   @override
   Widget build(BuildContext context) {
     return GestureDetector(
@@ -393,6 +394,7 @@ class FirstWidget extends StatelessWidget {
 }
 
 class SecondWidget extends StatelessWidget {
+  const SecondWidget({ Key key }) : super(key: key);
   @override
   Widget build(BuildContext context) {
     return const ModalBarrier(
@@ -403,6 +405,7 @@ class SecondWidget extends StatelessWidget {
 }
 
 class SecondWidgetWithCompetence extends StatelessWidget {
+  const SecondWidgetWithCompetence({ Key key }) : super(key: key);
   @override
   Widget build(BuildContext context) {
     return Stack(

packages/flutter/test/widgets/navigator_test.dart

@@ -12,6 +12,7 @@ import 'observer_tester.dart';
 import 'semantics_tester.dart';
 
 class FirstWidget extends StatelessWidget {
+  const FirstWidget({ Key key }) : super(key: key);
   @override
   Widget build(BuildContext context) {
     return GestureDetector(
@@ -27,6 +28,7 @@ class FirstWidget extends StatelessWidget {
 }
 
 class SecondWidget extends StatefulWidget {
+  const SecondWidget({ Key key }) : super(key: key);
   @override
   SecondWidgetState createState() => SecondWidgetState();
 }
@@ -47,7 +49,7 @@ class SecondWidgetState extends State<SecondWidget> {
 typedef ExceptionCallback = void Function(dynamic exception);
 
 class ThirdWidget extends StatelessWidget {
-  const ThirdWidget({ this.targetKey, this.onException });
+  const ThirdWidget({ Key key, this.targetKey, this.onException }) : super(key: key);
 
   final Key targetKey;
   final ExceptionCallback onException;
@@ -94,8 +96,8 @@ class OnTapPage extends StatelessWidget {
 void main() {
   testWidgets('Can navigator navigate to and from a stateful widget', (WidgetTester tester) async {
     final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
-      '/': (BuildContext context) => FirstWidget(), // X
-      '/second': (BuildContext context) => SecondWidget(), // Y
+      '/': (BuildContext context) => const FirstWidget(), // X
+      '/second': (BuildContext context) => const SecondWidget(), // Y
     };
 
     await tester.pumpWidget(MaterialApp(routes: routes));

packages/flutter/test/widgets/render_object_widget_test.dart

@@ -11,7 +11,10 @@ final BoxDecoration kBoxDecorationB = BoxDecoration(border: nonconst(null));
 final BoxDecoration kBoxDecorationC = BoxDecoration(border: nonconst(null));
 
 class TestWidget extends StatelessWidget {
-  const TestWidget({ this.child });
+  const TestWidget({
+    Key key,
+    this.child,
+  }) : super(key: key);
 
   final Widget child;
 

packages/flutter/test/widgets/reparent_state_harder_test.dart

@@ -57,7 +57,11 @@ class DummyStatefulWidgetState extends State<DummyStatefulWidget> {
 }
 
 class RekeyableDummyStatefulWidgetWrapper extends StatefulWidget {
-  const RekeyableDummyStatefulWidgetWrapper({ this.child, this.initialKey });
+  const RekeyableDummyStatefulWidgetWrapper({
+    Key key,
+    this.child,
+    this.initialKey,
+  }) : super(key: key);
   final Widget child;
   final GlobalKey initialKey;
   @override

packages/flutter/test/widgets/reparent_state_with_layout_builder_test.dart

@@ -10,6 +10,7 @@ import 'package:flutter_test/flutter_test.dart';
 // This is a regression test for https://github.com/flutter/flutter/issues/5840.
 
 class Bar extends StatefulWidget {
+  const Bar({ Key key }) : super(key: key);
   @override
   BarState createState() => BarState();
 }
@@ -68,7 +69,7 @@ class StatefulCreationCounterState extends State<StatefulCreationCounter> {
 void main() {
   testWidgets('reparent state with layout builder', (WidgetTester tester) async {
     expect(StatefulCreationCounterState.creationCount, 0);
-    await tester.pumpWidget(Bar());
+    await tester.pumpWidget(const Bar());
     expect(StatefulCreationCounterState.creationCount, 1);
     final BarState s = tester.state<BarState>(find.byType(Bar));
     s.trigger();

packages/flutter/test/widgets/scroll_position_test.dart

@@ -14,7 +14,10 @@ ScrollController _controller = ScrollController(
 );
 
 class ThePositiveNumbers extends StatelessWidget {
-  const ThePositiveNumbers({@required this.from});
+  const ThePositiveNumbers({
+    Key key,
+    @required this.from,
+  }) : super(key: key);
   final int from;
   @override
   Widget build(BuildContext context) {

packages/flutter/test/widgets/set_state_1_test.dart

@@ -6,6 +6,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
 class Inside extends StatefulWidget {
+  const Inside({ Key key }) : super(key: key);
   @override
   InsideState createState() => InsideState();
 }
@@ -25,7 +26,10 @@ class InsideState extends State<Inside> {
 }
 
 class Middle extends StatefulWidget {
-  const Middle({ this.child });
+  const Middle({
+    Key key,
+    this.child,
+  }) : super(key: key);
 
   final Inside child;
 
@@ -48,6 +52,7 @@ class MiddleState extends State<Middle> {
 }
 
 class Outside extends StatefulWidget {
+  const Outside({ Key key }) : super(key: key);
   @override
   OutsideState createState() => OutsideState();
 }
@@ -55,13 +60,13 @@ class Outside extends StatefulWidget {
 class OutsideState extends State<Outside> {
   @override
   Widget build(BuildContext context) {
-    return Middle(child: Inside());
+    return const Middle(child: Inside());
   }
 }
 
 void main() {
   testWidgets('setState() smoke test', (WidgetTester tester) async {
-    await tester.pumpWidget(Outside());
+    await tester.pumpWidget(const Outside());
     final Offset location = tester.getCenter(find.text('INSIDE'));
     final TestGesture gesture = await tester.startGesture(location);
     await tester.pump();

packages/flutter/test/widgets/set_state_3_test.dart

@@ -8,7 +8,7 @@ import 'package:flutter/widgets.dart';
 ChangerState changer;
 
 class Changer extends StatefulWidget {
-  const Changer(this.child);
+  const Changer(this.child, { Key key }) : super(key: key);
 
   final Widget child;
 
@@ -32,7 +32,7 @@ class ChangerState extends State<Changer> {
 }
 
 class Wrapper extends StatelessWidget {
-  const Wrapper(this.child);
+  const Wrapper(this.child, { Key key }) : super(key: key);
 
   final Widget child;
 
@@ -41,6 +41,7 @@ class Wrapper extends StatelessWidget {
 }
 
 class Leaf extends StatefulWidget {
+  const Leaf({ Key key }) : super(key: key);
   @override
   LeafState createState() => LeafState();
 }
@@ -52,8 +53,8 @@ class LeafState extends State<Leaf> {
 
 void main() {
   testWidgets('three-way setState() smoke test', (WidgetTester tester) async {
-    await tester.pumpWidget(Changer(Wrapper(Leaf())));
-    await tester.pumpWidget(Changer(Wrapper(Leaf())));
+    await tester.pumpWidget(const Changer(Wrapper(Leaf())));
+    await tester.pumpWidget(const Changer(Wrapper(Leaf())));
     changer.test();
     await tester.pump();
   });

packages/flutter/test/widgets/set_state_4_test.dart

@@ -6,6 +6,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
 class Changer extends StatefulWidget {
+  const Changer({ Key key }) : super(key: key);
   @override
   ChangerState createState() => ChangerState();
 }
@@ -21,7 +22,7 @@ class ChangerState extends State<Changer> {
 
 void main() {
   testWidgets('setState() catches being used with an async callback', (WidgetTester tester) async {
-    await tester.pumpWidget(Changer());
+    await tester.pumpWidget(const Changer());
     final ChangerState s = tester.state(find.byType(Changer));
     expect(s.test0, isNot(throwsFlutterError));
     expect(s.test1, isNot(throwsFlutterError));

packages/flutter/test/widgets/set_state_5_test.dart

@@ -6,9 +6,9 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
 class BadWidget extends StatefulWidget {
+  const BadWidget({ Key key }) : super(key: key);
   @override
   State<StatefulWidget> createState() => BadWidgetState();
-
 }
 
 class BadWidgetState extends State<BadWidget> {
@@ -28,7 +28,7 @@ class BadWidgetState extends State<BadWidget> {
 
 void main() {
   testWidgets('setState() catches being used inside a constructor', (WidgetTester tester) async {
-    await tester.pumpWidget(BadWidget());
+    await tester.pumpWidget(const BadWidget());
     expect(tester.takeException(), isInstanceOf<FlutterError>());
   });
 }

packages/flutter/test/widgets/size_changed_layout_notification_test.dart

@@ -5,14 +5,6 @@
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-class NotifyMaterial extends StatelessWidget {
-  @override
-  Widget build(BuildContext context) {
-    LayoutChangedNotification().dispatch(context);
-    return Container();
-  }
-}
-
 void main() {
   testWidgets('SizeChangedLayoutNotification test', (WidgetTester tester) async {
     bool notified = false;

packages/flutter/test/widgets/slivers_block_global_key_test.dart

@@ -9,7 +9,7 @@ import 'package:flutter/widgets.dart';
 int globalGeneration = 0;
 
 class GenerationText extends StatefulWidget {
-  const GenerationText(this.value);
+  const GenerationText(this.value, { Key key }) : super(key: key);
   final int value;
   @override
   _GenerationTextState createState() => _GenerationTextState();

packages/flutter/test/widgets/slivers_test.dart

@@ -701,7 +701,7 @@ bool sameHorizontal(Offset a, Offset b) => b.dy == a.dy;
 bool sameVertical(Offset a, Offset b) => b.dx == a.dx;
 
 class TestSliverGrid extends StatelessWidget {
-  const TestSliverGrid(this.children);
+  const TestSliverGrid(this.children, { Key key }) : super(key: key);
 
   final List<Widget> children;
 
@@ -726,7 +726,7 @@ class TestSliverGrid extends StatelessWidget {
 }
 
 class TestSliverFixedExtentList extends StatelessWidget {
-  const TestSliverFixedExtentList(this.children);
+  const TestSliverFixedExtentList(this.children, { Key key }) : super(key: key);
 
   final List<Widget> children;
 
@@ -749,7 +749,7 @@ class TestSliverFixedExtentList extends StatelessWidget {
 }
 
 class KeepAlive extends StatefulWidget {
-  const KeepAlive(this.data);
+  const KeepAlive(this.data, { Key key }) : super(key: key);
 
   final String data;
 

packages/flutter/test/widgets/state_setting_in_scrollables_test.dart

@@ -6,6 +6,7 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 class Foo extends StatefulWidget {
+  const Foo({ Key key }) : super(key: key);
   @override
   FooState createState() => FooState();
 }
@@ -80,7 +81,7 @@ class FooScrollBehavior extends ScrollBehavior {
 void main() {
   testWidgets('Can animate scroll after setState', (WidgetTester tester) async {
     await tester.pumpWidget(
-      Directionality(
+      const Directionality(
         textDirection: TextDirection.ltr,
         child: Foo(),
       ),

packages/flutter/test/widgets/stateful_component_test.dart

@@ -57,10 +57,10 @@ void main() {
 
   testWidgets('Don\'t rebuild subwidgets', (WidgetTester tester) async {
     await tester.pumpWidget(
-      FlipWidget(
-        key: const Key('rebuild test'),
+      const FlipWidget(
+        key: Key('rebuild test'),
         left: TestBuildCounter(),
-        right: const DecoratedBox(decoration: kBoxDecorationB),
+        right: DecoratedBox(decoration: kBoxDecorationB),
       ),
     );
 

packages/flutter/test/widgets/syncing_test.dart

@@ -6,7 +6,12 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
 class TestWidget extends StatefulWidget {
-  const TestWidget({ this.child, this.persistentState, this.syncedState });
+  const TestWidget({
+    Key key,
+    this.child,
+    this.persistentState,
+    this.syncedState,
+  }) : super(key: key);
 
   final Widget child;
   final int persistentState;

packages/flutter/test/widgets/test_widgets.dart

@@ -19,6 +19,8 @@ const BoxDecoration kBoxDecorationC = BoxDecoration(
 );
 
 class TestBuildCounter extends StatelessWidget {
+  const TestBuildCounter({ Key key }) : super(key: key);
+
   static int buildCount = 0;
 
   @override

packages/flutter/test/widgets/ticker_provider_test.dart

@@ -57,7 +57,7 @@ void main() {
   });
 
   testWidgets('SingleTickerProviderStateMixin can handle not being used', (WidgetTester tester) async {
-    final Widget widget = BoringTickerTest();
+    const Widget widget = BoringTickerTest();
     expect(widget.toString, isNot(throwsException));
 
     await tester.pumpWidget(widget);
@@ -228,6 +228,7 @@ void main() {
 }
 
 class BoringTickerTest extends StatefulWidget {
+  const BoringTickerTest({ Key key }) : super(key: key);
   @override
   _BoringTickerTestState createState() => _BoringTickerTestState();
 }

packages/flutter/test/widgets/widget_inspector_test.dart

@@ -18,6 +18,7 @@ import 'package:flutter_test/flutter_test.dart';
 // columns will impact whether tests pass.
 
 class ClockDemo extends StatefulWidget {
+  const ClockDemo({ Key key }) : super(key: key);
   @override
   _ClockDemoState createState() => _ClockDemoState();
 }
@@ -1634,7 +1635,7 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
     testWidgets('ext.flutter.inspector.trackRebuildDirtyWidgets', (WidgetTester tester) async {
       service.rebuildCount = 0;
 
-      await tester.pumpWidget(ClockDemo());
+      await tester.pumpWidget(const ClockDemo());
 
       final Element clockDemoElement = find.byType(ClockDemo).evaluate().first;
 
@@ -1724,7 +1725,7 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
       _CreationLocation location = knownLocations[id];
       expect(location.file, equals(file));
       // ClockText widget.
-      expect(location.line, equals(50));
+      expect(location.line, equals(51));
       expect(location.column, equals(9));
       expect(count, equals(1));
 
@@ -1733,7 +1734,7 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
       location = knownLocations[id];
       expect(location.file, equals(file));
       // Text widget in _ClockTextState build method.
-      expect(location.line, equals(88));
+      expect(location.line, equals(89));
       expect(location.column, equals(12));
       expect(count, equals(1));
 
@@ -1758,7 +1759,7 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
       location = knownLocations[id];
       expect(location.file, equals(file));
       // ClockText widget.
-      expect(location.line, equals(50));
+      expect(location.line, equals(51));
       expect(location.column, equals(9));
       expect(count, equals(3)); // 3 clock widget instances rebuilt.
 
@@ -1767,7 +1768,7 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
       location = knownLocations[id];
       expect(location.file, equals(file));
       // Text widget in _ClockTextState build method.
-      expect(location.line, equals(88));
+      expect(location.line, equals(89));
       expect(location.column, equals(12));
       expect(count, equals(3)); // 3 clock widget instances rebuilt.
 
@@ -1833,7 +1834,7 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
     testWidgets('ext.flutter.inspector.trackRepaintWidgets', (WidgetTester tester) async {
       service.rebuildCount = 0;
 
-      await tester.pumpWidget(ClockDemo());
+      await tester.pumpWidget(const ClockDemo());
 
       final Element clockDemoElement = find.byType(ClockDemo).evaluate().first;