Migrated some services and widgets doc comments to null safety. (#72792)

dev/tools/gen_keycodes/data/keyboard_key.tmpl

@@ -58,7 +58,7 @@ abstract class KeyboardKey with Diagnosticable {
 /// // The node used to request the keyboard focus.
 /// final FocusNode _focusNode = FocusNode();
 /// // The message to display.
-/// String _message;
+/// String? _message;
 ///
 /// // Focus nodes need to be disposed.
 /// @override
@@ -75,7 +75,7 @@ abstract class KeyboardKey with Diagnosticable {
 ///       _message = 'Pressed the "Q" key!';
 ///     } else {
 ///       if (kReleaseMode) {
-///         _message = 'Not a Q: Key label is "${event.logicalKey.keyLabel ?? '<none>'}"';
+///         _message = 'Not a Q: Key label is "${event.logicalKey.keyLabel}"';
 ///       } else {
 ///         // This will only print useful information in debug mode.
 ///         _message = 'Not a Q: Pressed ${event.logicalKey.debugName}';
@@ -91,13 +91,13 @@ abstract class KeyboardKey with Diagnosticable {
 ///     color: Colors.white,
 ///     alignment: Alignment.center,
 ///     child: DefaultTextStyle(
-///       style: textTheme.headline4,
+///       style: textTheme.headline4!,
 ///       child: RawKeyboardListener(
 ///         focusNode: _focusNode,
 ///         onKey: _handleKeyEvent,
 ///         child: AnimatedBuilder(
 ///           animation: _focusNode,
-///           builder: (BuildContext context, Widget child) {
+///           builder: (BuildContext context, Widget? child) {
 ///             if (!_focusNode.hasFocus) {
 ///               return GestureDetector(
 ///                 onTap: () {
@@ -346,7 +346,7 @@ class LogicalKeyboardKey extends KeyboardKey {
 /// // The node used to request the keyboard focus.
 /// final FocusNode _focusNode = FocusNode();
 /// // The message to display.
-/// String _message;
+/// String? _message;
 ///
 /// // Focus nodes need to be disposed.
 /// @override
@@ -374,13 +374,13 @@ class LogicalKeyboardKey extends KeyboardKey {
 ///     color: Colors.white,
 ///     alignment: Alignment.center,
 ///     child: DefaultTextStyle(
-///       style: textTheme.headline4,
+///       style: textTheme.headline4!,
 ///       child: RawKeyboardListener(
 ///         focusNode: _focusNode,
 ///         onKey: _handleKeyEvent,
 ///         child: AnimatedBuilder(
 ///           animation: _focusNode,
-///           builder: (BuildContext context, Widget child) {
+///           builder: (BuildContext context, Widget? child) {
 ///             if (!_focusNode.hasFocus) {
 ///               return GestureDetector(
 ///                 onTap: () {

packages/flutter/lib/src/services/keyboard_key.dart

@@ -45,7 +45,7 @@ abstract class KeyboardKey with Diagnosticable {
 /// look at the physical key to make sure that regardless of the character the
 /// key produces, you got the key that is in that location on the keyboard.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 /// This example shows how to detect if the user has selected the logical "Q"
 /// key.
 ///
@@ -58,7 +58,7 @@ abstract class KeyboardKey with Diagnosticable {
 /// // The node used to request the keyboard focus.
 /// final FocusNode _focusNode = FocusNode();
 /// // The message to display.
-/// String _message;
+/// String? _message;
 ///
 /// // Focus nodes need to be disposed.
 /// @override
@@ -75,7 +75,7 @@ abstract class KeyboardKey with Diagnosticable {
 ///       _message = 'Pressed the "Q" key!';
 ///     } else {
 ///       if (kReleaseMode) {
-///         _message = 'Not a Q: Key label is "${event.logicalKey.keyLabel ?? '<none>'}"';
+///         _message = 'Not a Q: Key label is "${event.logicalKey.keyLabel}"';
 ///       } else {
 ///         // This will only print useful information in debug mode.
 ///         _message = 'Not a Q: Pressed ${event.logicalKey.debugName}';
@@ -91,13 +91,13 @@ abstract class KeyboardKey with Diagnosticable {
 ///     color: Colors.white,
 ///     alignment: Alignment.center,
 ///     child: DefaultTextStyle(
-///       style: textTheme.headline4,
+///       style: textTheme.headline4!,
 ///       child: RawKeyboardListener(
 ///         focusNode: _focusNode,
 ///         onKey: _handleKeyEvent,
 ///         child: AnimatedBuilder(
 ///           animation: _focusNode,
-///           builder: (BuildContext context, Widget child) {
+///           builder: (BuildContext context, Widget? child) {
 ///             if (!_focusNode.hasFocus) {
 ///               return GestureDetector(
 ///                 onTap: () {
@@ -1987,7 +1987,7 @@ class LogicalKeyboardKey extends KeyboardKey {
 /// looking for "the key next next to the TAB key", since on a French keyboard,
 /// the key next to the TAB key has an "A" on it.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 /// This example shows how to detect if the user has selected the physical key
 /// to the right of the CAPS LOCK key.
 ///
@@ -1999,7 +1999,7 @@ class LogicalKeyboardKey extends KeyboardKey {
 /// // The node used to request the keyboard focus.
 /// final FocusNode _focusNode = FocusNode();
 /// // The message to display.
-/// String _message;
+/// String? _message;
 ///
 /// // Focus nodes need to be disposed.
 /// @override
@@ -2027,13 +2027,13 @@ class LogicalKeyboardKey extends KeyboardKey {
 ///     color: Colors.white,
 ///     alignment: Alignment.center,
 ///     child: DefaultTextStyle(
-///       style: textTheme.headline4,
+///       style: textTheme.headline4!,
 ///       child: RawKeyboardListener(
 ///         focusNode: _focusNode,
 ///         onKey: _handleKeyEvent,
 ///         child: AnimatedBuilder(
 ///           animation: _focusNode,
-///           builder: (BuildContext context, Widget child) {
+///           builder: (BuildContext context, Widget? child) {
 ///             if (!_focusNode.hasFocus) {
 ///               return GestureDetector(
 ///                 onTap: () {

packages/flutter/lib/src/services/platform_channel.dart

@@ -12,9 +12,6 @@ import 'binding.dart';
 import 'message_codec.dart';
 import 'message_codecs.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// A named channel for communicating with platform plugins using asynchronous
 /// message passing.
 ///
@@ -196,9 +193,9 @@ class MethodChannel {
   ///   static const MethodChannel _channel = MethodChannel('music');
   ///
   ///   static Future<bool> isLicensed() async {
-  ///     // invokeMethod returns a Future<T> which can be inferred as bool
-  ///     // in this context.
-  ///     return _channel.invokeMethod('isLicensed');
+  ///     // invokeMethod returns a Future<T?>, so we handle the case where
+  ///     // the return value is null by treating null as false.
+  ///     return _channel.invokeMethod<bool>('isLicensed').then<bool>((bool? value) => value ?? false);
   ///   }
   ///
   ///   static Future<List<Song>> songs() async {

packages/flutter/lib/src/services/system_chrome.dart

@@ -344,7 +344,7 @@ class SystemChrome {
   /// navigation bar and synthesize them into a single style. This can be used
   /// to configure the system styles when an app bar is not used.
   ///
-  /// {@tool sample --template=stateful_widget_material_no_null_safety}
+  /// {@tool sample --template=stateful_widget_material}
   /// The following example creates a widget that changes the status bar color
   /// to a random value on Android.
   ///

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

@@ -11,8 +11,7 @@ import 'ticker_provider.dart';
 import 'transitions.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// bool _first;
+// bool _first = false;
 
 /// Specifies which of two children to show. See [AnimatedCrossFade].
 ///

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

@@ -66,12 +66,11 @@ export 'package:flutter/rendering.dart' show
   WrapCrossAlignment;
 
 // Examples can assume:
-// // @dart = 2.9
 // class TestWidget extends StatelessWidget { @override Widget build(BuildContext context) => const Placeholder(); }
-// WidgetTester tester;
-// bool _visible;
+// late WidgetTester tester;
+// late bool _visible;
 // class Sky extends CustomPainter { @override void paint(Canvas c, Size s) => null; @override bool shouldRepaint(Sky s) => false; }
-// BuildContext context;
+// late BuildContext context;
 // dynamic userAvatarUrl;
 
 // BIDIRECTIONAL TEXT SUPPORT
@@ -1475,7 +1474,7 @@ class CompositedTransformFollower extends SingleChildRenderObjectWidget {
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=T4Uehk3_wlY}
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold_center}
 ///
 /// In this example, the image is stretched to fill the entire [Container], which would
 /// not happen normally without using FittedBox.
@@ -2759,7 +2758,7 @@ class SizedOverflowBox extends SingleChildRenderObjectWidget {
 /// needed, prefer removing the widget from the tree entirely rather than
 /// keeping it alive in an [Offstage] subtree.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center}
 ///
 /// This example shows a [FlutterLogo] widget when the `_offstage` member field
 /// is false, and hides it without any room in the parent when it is true. When
@@ -2771,7 +2770,7 @@ class SizedOverflowBox extends SingleChildRenderObjectWidget {
 /// bool _offstage = true;
 ///
 /// Size _getFlutterLogoSize() {
-///   final RenderBox renderLogo = _key.currentContext.findRenderObject();
+///   final RenderBox renderLogo = _key.currentContext!.findRenderObject()! as RenderBox;
 ///   return renderLogo.size;
 /// }
 ///
@@ -2883,7 +2882,7 @@ class _OffstageElement extends SingleChildRenderObjectElement {
 /// 16.0/9.0. If the maximum width is infinite, the initial width is determined
 /// by applying the aspect ratio to the maximum height.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 ///
 /// This examples shows how AspectRatio sets width when its parent's width
 /// constraint is infinite. Since its parent's allowed height is a fixed value,
@@ -2915,7 +2914,7 @@ class _OffstageElement extends SingleChildRenderObjectElement {
 /// the height to be between 0.0 and 100.0. We'll select a width of 100.0 (the
 /// biggest allowed) and a height of 50.0 (to match the aspect ratio).
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 ///
 /// ```dart
 /// Widget build(BuildContext context) {
@@ -2949,7 +2948,7 @@ class _OffstageElement extends SingleChildRenderObjectElement {
 /// will eventually select a size for the child that meets the layout
 /// constraints but fails to meet the aspect ratio constraints.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 ///
 /// ```dart
 /// Widget build(BuildContext context) {
@@ -4810,7 +4809,7 @@ class Flexible extends ParentDataWidget<FlexParentData> {
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=_rnZaagadyo}
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 /// This example shows how to use an [Expanded] widget in a [Column] so that
 /// its middle child, a [Container] here, expands to fill the space.
 ///
@@ -4849,7 +4848,7 @@ class Flexible extends ParentDataWidget<FlexParentData> {
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 /// This example shows how to use an [Expanded] widget in a [Row] with multiple
 /// children expanded, utilizing the [flex] factor to prioritize available space.
 ///
@@ -5200,7 +5199,7 @@ class Wrap extends MultiChildRenderObjectWidget {
 ///  * The [catalog of layout widgets](https://flutter.dev/widgets/layout/).
 ///
 ///
-/// {@tool dartpad --template=freeform_no_null_safety}
+/// {@tool dartpad --template=freeform}
 ///
 /// This example uses the [Flow] widget to create a menu that opens and closes
 /// as it is interacted with, shown above. The color of the button in the menu
@@ -5231,7 +5230,7 @@ class Wrap extends MultiChildRenderObjectWidget {
 /// }
 ///
 /// class _FlowMenuState extends State<FlowMenu> with SingleTickerProviderStateMixin {
-///   AnimationController menuAnimation;
+///   late AnimationController menuAnimation;
 ///   IconData lastTapped = Icons.notifications;
 ///   final List<IconData> menuItems = <IconData>[
 ///     Icons.home,
@@ -5291,7 +5290,7 @@ class Wrap extends MultiChildRenderObjectWidget {
 /// }
 ///
 /// class FlowMenuDelegate extends FlowDelegate {
-///   FlowMenuDelegate({this.menuAnimation}) : super(repaint: menuAnimation);
+///   FlowMenuDelegate({required this.menuAnimation}) : super(repaint: menuAnimation);
 ///
 ///   final Animation<double> menuAnimation;
 ///
@@ -5304,7 +5303,7 @@ class Wrap extends MultiChildRenderObjectWidget {
 ///   void paintChildren(FlowPaintingContext context) {
 ///     double dx = 0.0;
 ///     for (int i = 0; i < context.childCount; ++i) {
-///       dx = context.getChildSize(i).width * i;
+///       dx = context.getChildSize(i)!.width * i;
 ///       context.paintChild(
 ///         i,
 ///         transform: Matrix4.translationValues(
@@ -5843,7 +5842,7 @@ class RawImage extends LeafRenderObjectWidget {
 ///   Future<ByteData> load(String key) async {
 ///     if (key == 'resources/test')
 ///       return ByteData.view(Uint8List.fromList(utf8.encode('Hello World!')).buffer);
-///     return null;
+///     return ByteData(0);
 ///   }
 /// }
 /// ```
@@ -5968,7 +5967,7 @@ class WidgetToRenderBoxAdapter extends LeafRenderObjectWidget {
 /// If it has a child, this widget defers to the child for sizing behavior. If
 /// it does not have a child, it grows to fit the parent instead.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center}
 /// This example makes a [Container] react to being touched, showing a count of
 /// the number of pointer downs and ups.
 ///
@@ -6138,7 +6137,7 @@ class Listener extends SingleChildRenderObjectWidget {
 /// If it has a child, this widget defers to the child for sizing behavior. If
 /// it does not have a child, it grows to fit the parent instead.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center}
 /// This example makes a [Container] react to being entered by a mouse
 /// pointer, showing a count of the number of entries and exits.
 ///
@@ -6307,7 +6306,7 @@ class MouseRegion extends StatefulWidget {
   ///    override [State.dispose] and call [onExit], or create your own widget
   ///    using [RenderMouseRegion].
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_scaffold_center}
   /// The following example shows a blue rectangular that turns yellow when
   /// hovered. Since the hover state is completely contained within a widget
   /// that unconditionally creates the `MouseRegion`, you can ignore the
@@ -6335,7 +6334,7 @@ class MouseRegion extends StatefulWidget {
   /// ```
   /// {@end-tool}
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_scaffold_center}
   /// The following example shows a widget that hides its content one second
   /// after being hovered, and also exposes the enter and exit callbacks.
   /// Because the widget conditionally creates the `MouseRegion`, and leaks the
@@ -6347,7 +6346,7 @@ class MouseRegion extends StatefulWidget {
   /// ```dart preamble
   /// // A region that hides its content one second after being hovered.
   /// class MyTimedButton extends StatefulWidget {
-  ///   MyTimedButton({ Key key, this.onEnterButton, this.onExitButton })
+  ///   MyTimedButton({ Key? key, required this.onEnterButton, required this.onExitButton })
   ///     : super(key: key);
   ///
   ///   final VoidCallback onEnterButton;
@@ -6624,7 +6623,7 @@ class RepaintBoundary extends SingleChildRenderObjectWidget {
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=qV9pqHWxYgI}
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// The following sample has an [IgnorePointer] widget wrapping the `Column`
 /// which contains a button.
 /// When [ignoring] is set to `true` anything inside the `Column` can
@@ -6740,7 +6739,7 @@ class IgnorePointer extends SingleChildRenderObjectWidget {
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=65HoWqBboI8}
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold_center}
 /// The following sample has an [AbsorbPointer] widget wrapping the button on
 /// top of the stack, which absorbs pointer events, preventing its child button
 /// __and__ the button below it in the stack from receiving the pointer events.
@@ -7248,7 +7247,7 @@ class Semantics extends SingleChildRenderObjectWidget {
 ///     children: <Widget>[
 ///       Checkbox(
 ///         value: true,
-///         onChanged: (bool value) => null,
+///         onChanged: (bool? value) {},
 ///       ),
 ///       const Text("Settings"),
 ///     ],
@@ -7578,7 +7577,7 @@ typedef StatefulWidgetBuilder = Widget Function(BuildContext context, StateSette
 /// await showDialog<void>(
 ///   context: context,
 ///   builder: (BuildContext context) {
-///     int selectedRadio = 0;
+///     int? selectedRadio = 0;
 ///     return AlertDialog(
 ///       content: StatefulBuilder(
 ///         builder: (BuildContext context, StateSetter setState) {
@@ -7588,7 +7587,7 @@ typedef StatefulWidgetBuilder = Widget Function(BuildContext context, StateSette
 ///               return Radio<int>(
 ///                 value: index,
 ///                 groupValue: selectedRadio,
-///                 onChanged: (int value) {
+///                 onChanged: (int? value) {
 ///                   setState(() => selectedRadio = value);
 ///                 },
 ///               );

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

@@ -48,11 +48,10 @@ export 'package:flutter/gestures.dart' show
 export 'package:flutter/rendering.dart' show RenderSemanticsGestureHandler;
 
 // Examples can assume:
-// // @dart = 2.9
-// bool _lights;
+// late bool _lights;
 // void setState(VoidCallback fn) { }
-// String _last;
-// Color _color;
+// late String _last;
+// late Color _color;
 
 /// Factory for creating gesture recognizers.
 ///
@@ -1045,8 +1044,8 @@ class RawGestureDetector extends StatefulWidget {
   /// ```dart
   /// class ForcePressGestureDetectorWithSemantics extends StatelessWidget {
   ///   const ForcePressGestureDetectorWithSemantics({
-  ///     this.child,
-  ///     this.onForcePress,
+  ///     required this.child,
+  ///     required this.onForcePress,
   ///   });
   ///
   ///   final Widget child;

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

@@ -762,7 +762,7 @@ class Image extends StatefulWidget {
   /// ```
   /// {@endtemplate}
   ///
-  /// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_material}
   ///
   /// The following sample demonstrates how to use this builder to implement an
   /// image that fades in once it's been loaded.
@@ -781,8 +781,8 @@ class Image extends StatefulWidget {
   ///     ),
   ///     child: Image.network(
   ///       'https://flutter.github.io/assets-for-api-docs/assets/widgets/puffin.jpg',
-  ///       frameBuilder: (BuildContext context, Widget child, int frame, bool wasSynchronouslyLoaded) {
-  ///         if (wasSynchronouslyLoaded) {
+  ///       frameBuilder: (BuildContext context, Widget child, int? frame, bool? wasSynchronouslyLoaded) {
+  ///         if (wasSynchronouslyLoaded ?? false) {
   ///           return child;
   ///         }
   ///         return AnimatedOpacity(
@@ -828,7 +828,7 @@ class Image extends StatefulWidget {
   ///
   /// {@macro flutter.widgets.Image.frameBuilder.chainedBuildersExample}
   ///
-  /// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_material}
   ///
   /// The following sample uses [loadingBuilder] to show a
   /// [CircularProgressIndicator] while an image loads over the network.
@@ -843,13 +843,13 @@ class Image extends StatefulWidget {
   ///     ),
   ///     child: Image.network(
   ///       'https://example.com/image.jpg',
-  ///       loadingBuilder: (BuildContext context, Widget child, ImageChunkEvent loadingProgress) {
+  ///       loadingBuilder: (BuildContext context, Widget child, ImageChunkEvent? loadingProgress) {
   ///         if (loadingProgress == null)
   ///           return child;
   ///         return Center(
   ///           child: CircularProgressIndicator(
   ///             value: loadingProgress.expectedTotalBytes != null
-  ///                 ? loadingProgress.cumulativeBytesLoaded / loadingProgress.expectedTotalBytes
+  ///                 ? loadingProgress.cumulativeBytesLoaded / loadingProgress.expectedTotalBytes!
   ///                 : null,
   ///           ),
   ///         );
@@ -873,7 +873,7 @@ class Image extends StatefulWidget {
   /// [FlutterError.onError]. If it is provided, the caller should either handle
   /// the exception by providing a replacement widget, or rethrow the exception.
   ///
-  /// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_material}
   ///
   /// The following sample uses [errorBuilder] to show a '😢' in place of the
   /// image that fails to load, and prints the error to the console.
@@ -888,7 +888,7 @@ class Image extends StatefulWidget {
   ///     ),
   ///     child: Image.network(
   ///       'https://example.does.not.exist/image.jpg',
-  ///       errorBuilder: (BuildContext context, Object exception, StackTrace stackTrace) {
+  ///       errorBuilder: (BuildContext context, Object exception, StackTrace? stackTrace) {
   ///         // Appropriate logging or analytics, e.g.
   ///         // myAnalytics.recordError(
   ///         //   'An error occurred loading "https://example.does.not.exist/image.jpg"',

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

@@ -19,7 +19,6 @@ import 'ticker_provider.dart';
 import 'transitions.dart';
 
 // Examples can assume:
-// // @dart = 2.9
 // class MyWidget extends ImplicitlyAnimatedWidget {
 //   MyWidget() : super(duration: const Duration(seconds: 1));
 //   final Color targetColor = Colors.black;
@@ -480,14 +479,14 @@ abstract class ImplicitlyAnimatedWidgetState<T extends ImplicitlyAnimatedWidget>
   ///
   /// ```dart
   /// class MyWidgetState extends AnimatedWidgetBaseState<MyWidget> {
-  ///   ColorTween _colorTween;
+  ///   ColorTween? _colorTween;
   ///
   ///   @override
   ///   Widget build(BuildContext context) {
   ///     return Text(
   ///       'Hello World',
   ///       // Computes the value of the text color at any given time.
-  ///       style: TextStyle(color: _colorTween.evaluate(animation)),
+  ///       style: TextStyle(color: _colorTween?.evaluate(animation)),
   ///     );
   ///   }
   ///
@@ -502,7 +501,7 @@ abstract class ImplicitlyAnimatedWidgetState<T extends ImplicitlyAnimatedWidget>
   ///       // A function that takes a color value and returns a tween
   ///       // beginning at that value.
   ///       (value) => ColorTween(begin: value),
-  ///     );
+  ///     ) as ColorTween?;
   ///
   ///     // We could have more tweens than one by using the visitor
   ///     // multiple times.
@@ -573,7 +572,7 @@ abstract class AnimatedWidgetBaseState<T extends ImplicitlyAnimatedWidget> exten
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=yI-8QHpGIP4}
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 ///
 /// The following example (depicted above) transitions an AnimatedContainer
 /// between two states. It adjusts the `height`, `width`, `color`, and
@@ -812,7 +811,7 @@ class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer>
 /// of [Curves.fastOutSlowIn].
 /// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_padding.mp4}
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 ///
 /// The following code implements the [AnimatedPadding] widget, using a [curve] of
 /// [Curves.easeInOut].
@@ -933,7 +932,7 @@ class _AnimatedPaddingState extends AnimatedWidgetBaseState<AnimatedPadding> {
 /// it also requires more development overhead as you have to manually manage
 /// the lifecycle of the underlying [AnimationController].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 ///
 /// The following code implements the [AnimatedAlign] widget, using a [curve] of
 /// [Curves.fastOutSlowIn].
@@ -1098,7 +1097,7 @@ class _AnimatedAlignState extends AnimatedWidgetBaseState<AnimatedAlign> {
 /// it also requires more development overhead as you have to manually manage
 /// the lifecycle of the underlying [AnimationController].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center}
 ///
 /// The following example transitions an AnimatedPositioned
 /// between two states. It adjusts the `height`, `width`, and
@@ -1550,7 +1549,7 @@ class _AnimatedOpacityState extends ImplicitlyAnimatedWidgetState<AnimatedOpacit
 /// Here's an illustration of what using this widget looks like, using a [curve]
 /// of [Curves.fastOutSlowIn].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_freeform_state_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_freeform_state}
 /// Creates a [CustomScrollView] with a [SliverFixedExtentList] and a
 /// [FloatingActionButton]. Pressing the button animates the lists' opacity.
 ///

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

@@ -26,11 +26,10 @@ import 'routes.dart';
 import 'ticker_provider.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// class MyPage extends Placeholder { MyPage({String title}); }
+// class MyPage extends Placeholder { MyPage({String? title}); }
 // class MyHomePage extends Placeholder { }
-// NavigatorState navigator;
-// BuildContext context;
+// late NavigatorState navigator;
+// late BuildContext context;
 
 /// Creates a route for the given route settings.
 ///
@@ -734,7 +733,7 @@ abstract class RouteTransitionRecord {
 ///
 /// To make route transition decisions, subclass must implement [resolve].
 ///
-/// {@tool sample --template=freeform_no_null_safety}
+/// {@tool sample --template=freeform}
 /// The following example demonstrates how to implement a subclass that always
 /// removes or adds routes without animated transitions and puts the removed
 /// routes at the top of the list.
@@ -747,9 +746,9 @@ abstract class RouteTransitionRecord {
 /// class NoAnimationTransitionDelegate extends TransitionDelegate<void> {
 ///   @override
 ///   Iterable<RouteTransitionRecord> resolve({
-///     List<RouteTransitionRecord> newPageRouteHistory,
-///     Map<RouteTransitionRecord, RouteTransitionRecord> locationToExitingPageRoute,
-///     Map<RouteTransitionRecord, List<RouteTransitionRecord>> pageRouteToPagelessRoutes,
+///     required List<RouteTransitionRecord> newPageRouteHistory,
+///     required Map<RouteTransitionRecord?, RouteTransitionRecord> locationToExitingPageRoute,
+///     required Map<RouteTransitionRecord?, List<RouteTransitionRecord>> pageRouteToPagelessRoutes,
 ///   }) {
 ///     final List<RouteTransitionRecord> results = <RouteTransitionRecord>[];
 ///
@@ -763,7 +762,7 @@ abstract class RouteTransitionRecord {
 ///     for (final RouteTransitionRecord exitingPageRoute in locationToExitingPageRoute.values) {
 ///       if (exitingPageRoute.isWaitingForExitingDecision) {
 ///        exitingPageRoute.markForRemove();
-///        final List<RouteTransitionRecord> pagelessRoutes = pageRouteToPagelessRoutes[exitingPageRoute];
+///        final List<RouteTransitionRecord>? pagelessRoutes = pageRouteToPagelessRoutes[exitingPageRoute];
 ///        if (pagelessRoutes != null) {
 ///          for (final RouteTransitionRecord pagelessRoute in pagelessRoutes) {
 ///             pagelessRoute.markForRemove();
@@ -1267,7 +1266,7 @@ class DefaultTransitionDelegate<T> extends TransitionDelegate<T> {
 /// [WidgetsApp] and [CupertinoTabView] widgets and do not need to be explicitly
 /// created or managed.
 ///
-/// {@tool sample --template=freeform_no_null_safety}
+/// {@tool sample --template=freeform}
 /// The following example demonstrates how a nested [Navigator] can be used to
 /// present a standalone user registration journey.
 ///
@@ -1305,7 +1304,7 @@ class DefaultTransitionDelegate<T> extends TransitionDelegate<T> {
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return DefaultTextStyle(
-///       style: Theme.of(context).textTheme.headline4,
+///       style: Theme.of(context).textTheme.headline4!,
 ///       child: Container(
 ///         color: Colors.white,
 ///         alignment: Alignment.center,
@@ -1319,7 +1318,7 @@ class DefaultTransitionDelegate<T> extends TransitionDelegate<T> {
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return DefaultTextStyle(
-///       style: Theme.of(context).textTheme.headline4,
+///       style: Theme.of(context).textTheme.headline4!,
 ///       child: GestureDetector(
 ///         onTap: () {
 ///           // This moves from the personal info page to the credentials page,
@@ -1339,7 +1338,7 @@ class DefaultTransitionDelegate<T> extends TransitionDelegate<T> {
 ///
 /// class ChooseCredentialsPage extends StatelessWidget {
 ///   const ChooseCredentialsPage({
-///     this.onSignupComplete,
+///     required this.onSignupComplete,
 ///   });
 ///
 ///   final VoidCallback onSignupComplete;
@@ -1349,7 +1348,7 @@ class DefaultTransitionDelegate<T> extends TransitionDelegate<T> {
 ///     return GestureDetector(
 ///       onTap: onSignupComplete,
 ///       child: DefaultTextStyle(
-///         style: Theme.of(context).textTheme.headline4,
+///         style: Theme.of(context).textTheme.headline4!,
 ///         child: Container(
 ///           color: Colors.pinkAccent,
 ///           alignment: Alignment.center,
@@ -1674,7 +1673,7 @@ class Navigator extends StatefulWidget {
   ///
   /// ```dart
   /// class WeatherRouteArguments {
-  ///   WeatherRouteArguments({ this.city, this.country });
+  ///   WeatherRouteArguments({ required this.city, required this.country });
   ///   final String city;
   ///   final String country;
   ///
@@ -2120,12 +2119,12 @@ class Navigator extends StatefulWidget {
   ///
   /// {@macro flutter.widgets.Navigator.restorablePushNamed.returnValue}
   ///
-  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material}
   ///
   /// Typical usage is as follows:
   ///
   /// ```dart
-  /// static Route _myRouteBuilder(BuildContext context, Object arguments) {
+  /// static Route _myRouteBuilder(BuildContext context, Object? arguments) {
   ///   return MaterialPageRoute(
   ///     builder: (BuildContext context) => MyStatefulWidget(),
   ///   );
@@ -2215,12 +2214,12 @@ class Navigator extends StatefulWidget {
   ///
   /// {@macro flutter.widgets.Navigator.restorablePushNamed.returnValue}
   ///
-  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material}
   ///
   /// Typical usage is as follows:
   ///
   /// ```dart
-  /// static Route _myRouteBuilder(BuildContext context, Object arguments) {
+  /// static Route _myRouteBuilder(BuildContext context, Object? arguments) {
   ///   return MaterialPageRoute(
   ///     builder: (BuildContext context) => MyStatefulWidget(),
   ///   );
@@ -2320,12 +2319,12 @@ class Navigator extends StatefulWidget {
   ///
   /// {@macro flutter.widgets.Navigator.restorablePushNamed.returnValue}
   ///
-  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material}
   ///
   /// Typical usage is as follows:
   ///
   /// ```dart
-  /// static Route _myRouteBuilder(BuildContext context, Object arguments) {
+  /// static Route _myRouteBuilder(BuildContext context, Object? arguments) {
   ///   return MaterialPageRoute(
   ///     builder: (BuildContext context) => MyStatefulWidget(),
   ///   );
@@ -4364,12 +4363,12 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
   ///
   /// {@macro flutter.widgets.Navigator.restorablePushNamed.returnValue}
   ///
-  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material}
   ///
   /// Typical usage is as follows:
   ///
   /// ```dart
-  /// static Route _myRouteBuilder(BuildContext context, Object arguments) {
+  /// static Route _myRouteBuilder(BuildContext context, Object? arguments) {
   ///   return MaterialPageRoute(
   ///     builder: (BuildContext context) => MyStatefulWidget(),
   ///   );
@@ -4502,12 +4501,12 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
   ///
   /// {@macro flutter.widgets.Navigator.restorablePushNamed.returnValue}
   ///
-  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material}
   ///
   /// Typical usage is as follows:
   ///
   /// ```dart
-  /// static Route _myRouteBuilder(BuildContext context, Object arguments) {
+  /// static Route _myRouteBuilder(BuildContext context, Object? arguments) {
   ///   return MaterialPageRoute(
   ///     builder: (BuildContext context) => MyStatefulWidget(),
   ///   );
@@ -4608,12 +4607,12 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
   ///
   /// {@macro flutter.widgets.Navigator.restorablePushNamed.returnValue}
   ///
-  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material}
   ///
   /// Typical usage is as follows:
   ///
   /// ```dart
-  /// static Route _myRouteBuilder(BuildContext context, Object arguments) {
+  /// static Route _myRouteBuilder(BuildContext context, Object? arguments) {
   ///   return MaterialPageRoute(
   ///     builder: (BuildContext context) => MyStatefulWidget(),
   ///   );
@@ -5528,7 +5527,7 @@ typedef RouteCompletionCallback<T> = void Function(T result);
 /// When [present] has been called to add a route, it may only be called again
 /// after the previously added route has completed.
 ///
-/// {@tool dartpad --template=freeform_no_null_safety}
+/// {@tool dartpad --template=freeform}
 /// This example uses a [RestorableRouteFuture] in the `_MyHomeState` to push a
 /// new `MyCounter` route and to retrieve its return value.
 ///
@@ -5557,7 +5556,7 @@ typedef RouteCompletionCallback<T> = void Function(T result);
 ///
 /// ```dart
 /// class MyHome extends StatefulWidget {
-///   const MyHome({Key key}) : super(key: key);
+///   const MyHome({Key? key}) : super(key: key);
 ///
 ///   @override
 ///   State<MyHome> createState() => _MyHomeState();
@@ -5565,7 +5564,7 @@ typedef RouteCompletionCallback<T> = void Function(T result);
 ///
 /// class _MyHomeState extends State<MyHome> with RestorationMixin {
 ///   final RestorableInt _lastCount = RestorableInt(0);
-///   RestorableRouteFuture<int> _counterRoute;
+///   late RestorableRouteFuture<int> _counterRoute;
 ///
 ///   @override
 ///   String get restorationId => 'home';
@@ -5573,7 +5572,7 @@ typedef RouteCompletionCallback<T> = void Function(T result);
 ///   void initState() {
 ///     super.initState();
 ///     _counterRoute = RestorableRouteFuture<int>(
-///       onPresent: (NavigatorState navigator, Object arguments) {
+///       onPresent: (NavigatorState navigator, Object? arguments) {
 ///         // Defines what route should be shown (and how it should be added
 ///         // to the navigator) when `RestorableRouteFuture.present` is called.
 ///         return navigator.restorablePush(
@@ -5592,7 +5591,7 @@ typedef RouteCompletionCallback<T> = void Function(T result);
 ///   }
 ///
 ///   @override
-///   void restoreState(RestorationBucket oldBucket, bool initialRestore) {
+///   void restoreState(RestorationBucket? oldBucket, bool initialRestore) {
 ///     // Register the `RestorableRouteFuture` with the state restoration framework.
 ///     registerForRestoration(_counterRoute, 'route');
 ///     registerForRestoration(_lastCount, 'count');
@@ -5602,12 +5601,12 @@ typedef RouteCompletionCallback<T> = void Function(T result);
 ///   void dispose() {
 ///     super.dispose();
 ///     _lastCount.dispose();
-///     _counterRoute?.dispose();
+///     _counterRoute.dispose();
 ///   }
 ///
 ///   // A static `RestorableRouteBuilder` that can re-create the route during
 ///   // state restoration.
-///   static Route<int> _counterRouteBuilder(BuildContext context, Object arguments) {
+///   static Route<int> _counterRouteBuilder(BuildContext context, Object? arguments) {
 ///     return MaterialPageRoute(
 ///       builder: (BuildContext context) => MyCounter(
 ///         title: arguments as String,
@@ -5637,7 +5636,7 @@ typedef RouteCompletionCallback<T> = void Function(T result);
 ///
 /// // Widget for the route pushed by the `RestorableRouteFuture`.
 /// class MyCounter extends StatefulWidget {
-///   const MyCounter({Key key, this.title}) : super(key: key);
+///   const MyCounter({Key? key, required this.title}) : super(key: key);
 ///
 ///   final String title;
 ///
@@ -5652,7 +5651,7 @@ typedef RouteCompletionCallback<T> = void Function(T result);
 ///   String get restorationId => 'counter';
 ///
 ///   @override
-///   void restoreState(RestorationBucket oldBucket, bool initialRestore) {
+///   void restoreState(RestorationBucket? oldBucket, bool initialRestore) {
 ///     registerForRestoration(_count, 'count');
 ///   }
 ///

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

@@ -10,9 +10,6 @@ import 'package:flutter/services.dart';
 import 'editable_text.dart';
 import 'restoration.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// A [RestorableProperty] that makes the wrapped value accessible to the owning
 /// [State] object via the [value] getter and setter.
 ///
@@ -22,7 +19,7 @@ import 'restoration.dart';
 ///
 /// ## Using a RestorableValue
 ///
-/// {@tool dartpad --template=stateful_widget_restoration_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_restoration}
 /// A [StatefulWidget] that has a restorable [int] property.
 ///
 /// ```dart
@@ -33,7 +30,7 @@ import 'restoration.dart';
 ///   RestorableInt _answer = RestorableInt(42);
 ///
 ///   @override
-///   void restoreState(RestorationBucket oldBucket, bool initialRestore) {
+///   void restoreState(RestorationBucket? oldBucket, bool initialRestore) {
 ///     // All restorable properties must be registered with the mixin. After
 ///     // registration, the answer either has its old value restored or is
 ///     // initialized to its default value.
@@ -77,15 +74,18 @@ import 'restoration.dart';
 ///   Duration createDefaultValue() => const Duration();
 ///
 ///   @override
-///   void didUpdateValue(Duration oldValue) {
-///     if (oldValue.inMicroseconds != value.inMicroseconds)
+///   void didUpdateValue(Duration? oldValue) {
+///     if (oldValue == null || oldValue.inMicroseconds != value.inMicroseconds)
 ///       notifyListeners();
 ///   }
 ///
 ///   @override
-///   Duration fromPrimitives(Object data) {
+///   Duration fromPrimitives(Object? data) {
+///     if (data != null) {
 ///       return Duration(microseconds: data as int);
 ///     }
+///     return const Duration();
+///   }
 ///
 ///   @override
 ///   Object toPrimitives() {

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

@@ -24,9 +24,8 @@ import 'scroll_controller.dart';
 import 'transitions.dart';
 
 // Examples can assume:
-// // @dart = 2.9
 // dynamic routeObserver;
-// NavigatorState navigator;
+// late NavigatorState navigator;
 
 /// A route that displays widgets in the [Navigator]'s [Overlay].
 abstract class OverlayRoute<T> extends Route<T> {
@@ -541,7 +540,7 @@ mixin LocalHistoryRoute<T> on Route<T> {
   ///     // rectangle. When this local history entry is removed, we hide the red
   ///     // rectangle.
   ///     setState(() => _showRectangle = true);
-  ///     ModalRoute.of(context).addLocalHistoryEntry(
+  ///     ModalRoute.of(context)?.addLocalHistoryEntry(
   ///         LocalHistoryEntry(
   ///             onRemove: () {
   ///               // Hide the red rectangle.

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

@@ -13,8 +13,7 @@ import 'inherited_theme.dart';
 import 'media_query.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// String _name;
+// late String _name;
 
 /// The text style to apply to descendant [Text] widgets which don't have an
 /// explicit style.