diff --git a/packages/flutter/lib/src/widgets/actions.dart b/packages/flutter/lib/src/widgets/actions.dart
index 21468e5ed1..18afa116a3 100644
--- a/packages/flutter/lib/src/widgets/actions.dart
+++ b/packages/flutter/lib/src/widgets/actions.dart
@@ -795,7 +795,7 @@ class _ActionsMarker extends InheritedWidget {
 /// widget, and the new control should be enabled for keyboard traversal and
 /// activation.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// This example shows how keyboard interaction can be added to a custom control
 /// that changes color when hovered and focused, and can toggle a light when
 /// activated, either by touch or by hitting the `X` key on the keyboard when
@@ -813,7 +813,11 @@ class _ActionsMarker extends InheritedWidget {
 ///
 /// ```dart preamble
 /// class FadButton extends StatefulWidget {
-///   const FadButton({Key key, this.onPressed, this.child}) : super(key: key);
+///   const FadButton({
+///     Key? key,
+///     required this.onPressed,
+///     required this.child,
+///   }) : super(key: key);
 ///
 ///   final VoidCallback onPressed;
 ///   final Widget child;
@@ -826,8 +830,8 @@ class _ActionsMarker extends InheritedWidget {
 ///   bool _focused = false;
 ///   bool _hovering = false;
 ///   bool _on = false;
-///   Map<Type, Action<Intent>> _actionMap;
-///   Map<LogicalKeySet, Intent> _shortcutMap;
+///   late Map<Type, Action<Intent>> _actionMap;
+///   late Map<LogicalKeySet, Intent> _shortcutMap;
 ///
 ///   @override
 ///   void initState() {
diff --git a/packages/flutter/lib/src/widgets/async.dart b/packages/flutter/lib/src/widgets/async.dart
index 6156b4fa1b..4d13b9c9df 100644
--- a/packages/flutter/lib/src/widgets/async.dart
+++ b/packages/flutter/lib/src/widgets/async.dart
@@ -12,11 +12,6 @@ import 'package:flutter/foundation.dart';
 
 import 'framework.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-// dynamic _lot;
-// Future<String> _calculation;
-
 /// Base class for widgets that build themselves based on interaction with
 /// a specified [Stream].
 ///
@@ -371,7 +366,7 @@ typedef AsyncWidgetBuilder<T> = Widget Function(BuildContext context, AsyncSnaps
 /// as the builder will always be called before the stream listener has a chance
 /// to be processed.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 ///
 /// This sample shows a [StreamBuilder] that listens to a Stream that emits bids
 /// for an auction. Every time the StreamBuilder receives a bid from the Stream,
@@ -388,7 +383,7 @@ typedef AsyncWidgetBuilder<T> = Widget Function(BuildContext context, AsyncSnaps
 ///
 /// Widget build(BuildContext context) {
 ///   return DefaultTextStyle(
-///     style: Theme.of(context).textTheme.headline2,
+///     style: Theme.of(context).textTheme.headline2!,
 ///     textAlign: TextAlign.center,
 ///     child: Container(
 ///       alignment: FractionalOffset.center,
@@ -614,7 +609,7 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
 /// `future?.asStream()`, except that snapshots with `ConnectionState.active`
 /// may appear for the latter, depending on how the stream is implemented.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 ///
 /// This sample shows a [FutureBuilder] that displays a loading spinner while it
 /// loads data. It displays a success icon and text if the [Future] completes
@@ -630,7 +625,7 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
 ///
 /// Widget build(BuildContext context) {
 ///   return DefaultTextStyle(
-///     style: Theme.of(context).textTheme.headline2,
+///     style: Theme.of(context).textTheme.headline2!,
 ///     textAlign: TextAlign.center,
 ///     child: FutureBuilder<String>(
 ///       future: _calculation, // a previously-obtained Future<String> or null
diff --git a/packages/flutter/lib/src/widgets/form.dart b/packages/flutter/lib/src/widgets/form.dart
index 0d6412748d..66e44de662 100644
--- a/packages/flutter/lib/src/widgets/form.dart
+++ b/packages/flutter/lib/src/widgets/form.dart
@@ -16,7 +16,7 @@ import 'will_pop_scope.dart';
 /// with a context whose ancestor is the [Form], or pass a [GlobalKey] to the
 /// [Form] constructor and call [GlobalKey.currentState].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 /// This example shows a [Form] with one [TextFormField] to enter an email
 /// address and an [ElevatedButton] to submit the form. A [GlobalKey] is used here
 /// to identify the [Form] and validate input.
@@ -38,7 +38,7 @@ import 'will_pop_scope.dart';
 ///             hintText: 'Enter your email',
 ///           ),
 ///           validator: (value) {
-///             if (value.isEmpty) {
+///             if (value!.isEmpty) {
 ///               return 'Please enter some text';
 ///             }
 ///             return null;
@@ -50,7 +50,7 @@ import 'will_pop_scope.dart';
 ///             onPressed: () {
 ///               // Validate will return true if the form is valid, or false if
 ///               // the form is invalid.
-///               if (_formKey.currentState.validate()) {
+///               if (_formKey.currentState!.validate()) {
 ///                 // Process data.
 ///               }
 ///             },
diff --git a/packages/flutter/lib/src/widgets/page_storage.dart b/packages/flutter/lib/src/widgets/page_storage.dart
index 9ea84c02a4..f8284b638a 100644
--- a/packages/flutter/lib/src/widgets/page_storage.dart
+++ b/packages/flutter/lib/src/widgets/page_storage.dart
@@ -136,7 +136,7 @@ class PageStorageBucket {
 /// you should give each of them unique [PageStorageKey]s, or set some of their
 /// `keepScrollOffset` false to prevent saving.
 ///
-/// {@tool dartpad --template=freeform_no_null_safety}
+/// {@tool dartpad --template=freeform}
 ///
 /// This sample shows how to explicitly use a [PageStorage] to
 /// store the states of its children pages. Each page includes a scrollable
@@ -212,7 +212,7 @@ class PageStorageBucket {
 ///
 /// class ColorBoxPage extends StatelessWidget {
 ///   ColorBoxPage({
-///     Key key,
+///     Key? key,
 ///   }) : super(key: key);
 ///
 ///   @override
diff --git a/packages/flutter/lib/src/widgets/single_child_scroll_view.dart b/packages/flutter/lib/src/widgets/single_child_scroll_view.dart
index b15b0c799d..c5ab2db9c5 100644
--- a/packages/flutter/lib/src/widgets/single_child_scroll_view.dart
+++ b/packages/flutter/lib/src/widgets/single_child_scroll_view.dart
@@ -80,7 +80,7 @@ import 'scrollable.dart';
 /// with some remaining space to allocate as specified by its
 /// [Column.mainAxisAlignment] argument.
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 /// In this example, the children are spaced out equally, unless there's no more
 /// room, in which case they stack vertically and scroll.
 ///
@@ -91,7 +91,7 @@ import 'scrollable.dart';
 /// ```dart
 ///  Widget build(BuildContext context) {
 ///    return DefaultTextStyle(
-///      style: Theme.of(context).textTheme.bodyText2,
+///      style: Theme.of(context).textTheme.bodyText2!,
 ///      child: LayoutBuilder(
 ///        builder: (BuildContext context, BoxConstraints viewportConstraints) {
 ///          return SingleChildScrollView(
@@ -154,14 +154,14 @@ import 'scrollable.dart';
 /// so that the intrinsic sizing algorithm can short-circuit the computation when it
 /// reaches those parts of the subtree.
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 /// In this example, the column becomes either as big as viewport, or as big as
 /// the contents, whichever is biggest.
 ///
 /// ```dart
 ///  Widget build(BuildContext context) {
 ///    return DefaultTextStyle(
-///      style: Theme.of(context).textTheme.bodyText2,
+///      style: Theme.of(context).textTheme.bodyText2!,
 ///      child: LayoutBuilder(
 ///        builder: (BuildContext context, BoxConstraints viewportConstraints) {
 ///          return SingleChildScrollView(
diff --git a/packages/flutter/lib/src/widgets/transitions.dart b/packages/flutter/lib/src/widgets/transitions.dart
index 2d958a9673..73eb57ba85 100644
--- a/packages/flutter/lib/src/widgets/transitions.dart
+++ b/packages/flutter/lib/src/widgets/transitions.dart
@@ -25,7 +25,7 @@ export 'package:flutter/rendering.dart' show RelativeRect;
 /// [AnimatedWidget] is most useful for widgets that are otherwise stateless. To
 /// use [AnimatedWidget], simply subclass it and implement the build function.
 ///
-///{@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// This code defines a widget called `Spinner` that spins a green square
 /// continually. It is built with an [AnimatedWidget].
@@ -36,10 +36,12 @@ export 'package:flutter/rendering.dart' show RelativeRect;
 ///
 /// ```dart preamble
 /// class SpinningContainer extends AnimatedWidget {
-///   const SpinningContainer({Key key, AnimationController controller})
-///       : super(key: key, listenable: controller);
+///   const SpinningContainer({
+///     Key? key,
+///     required AnimationController controller,
+///   }) : super(key: key, listenable: controller);
 ///
-///   Animation<double> get _progress => listenable;
+///   Animation<double> get _progress => listenable as Animation<double>;
 ///
 ///   @override
 ///   Widget build(BuildContext context) {
@@ -52,16 +54,10 @@ export 'package:flutter/rendering.dart' show RelativeRect;
 /// ```
 ///
 /// ```dart
-/// AnimationController _controller;
-///
-/// @override
-/// void initState() {
-///   super.initState();
-///   _controller = AnimationController(
-///     duration: const Duration(seconds: 10),
-///     vsync: this,
-///   )..repeat();
-/// }
+/// late AnimationController _controller = AnimationController(
+///   duration: const Duration(seconds: 10),
+///   vsync: this,
+/// )..repeat();
 ///
 /// @override
 /// void dispose() {
@@ -195,30 +191,23 @@ class _AnimatedState extends State<AnimatedWidget> {
 /// animated by a [CurvedAnimation] set to [Curves.elasticIn]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/slide_transition.mp4}
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_freeform_state_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_freeform_state}
 /// The following code implements the [SlideTransition] as seen in the video
 /// above:
 ///
 /// ```dart
 /// class _MyStatefulWidgetState extends State<MyStatefulWidget> with SingleTickerProviderStateMixin {
-///   AnimationController _controller;
-///   Animation<Offset> _offsetAnimation;
-///
-///   @override
-///   void initState() {
-///     super.initState();
-///     _controller = AnimationController(
-///       duration: const Duration(seconds: 2),
-///       vsync: this,
-///     )..repeat(reverse: true);
-///     _offsetAnimation = Tween<Offset>(
-///       begin: Offset.zero,
-///       end: const Offset(1.5, 0.0),
-///     ).animate(CurvedAnimation(
-///       parent: _controller,
-///       curve: Curves.elasticIn,
-///     ));
-///   }
+///   late AnimationController _controller = AnimationController(
+///     duration: const Duration(seconds: 2),
+///     vsync: this,
+///   )..repeat(reverse: true);
+///   late Animation<Offset> _offsetAnimation = Tween<Offset>(
+///     begin: Offset.zero,
+///     end: const Offset(1.5, 0.0),
+///   ).animate(CurvedAnimation(
+///     parent: _controller,
+///     curve: Curves.elasticIn,
+///   ));
 ///
 ///   @override
 ///   void dispose() {
@@ -313,27 +302,20 @@ class SlideTransition extends AnimatedWidget {
 /// animated by a [CurvedAnimation] set to [Curves.fastOutSlowIn]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/scale_transition.mp4}
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// The following code implements the [ScaleTransition] as seen in the video
 /// above:
 ///
 /// ```dart
-/// AnimationController _controller;
-/// Animation<double> _animation;
-///
-/// @override
-/// void initState() {
-///   super.initState();
-///   _controller = AnimationController(
-///     duration: const Duration(seconds: 2),
-///     vsync: this,
-///   )..repeat(reverse: true);
-///   _animation = CurvedAnimation(
-///     parent: _controller,
-///     curve: Curves.fastOutSlowIn,
-///   );
-/// }
+/// late AnimationController _controller = AnimationController(
+///   duration: const Duration(seconds: 2),
+///   vsync: this,
+/// )..repeat(reverse: true);
+/// late Animation<double> _animation = CurvedAnimation(
+///   parent: _controller,
+///   curve: Curves.fastOutSlowIn,
+/// );
 ///
 /// @override
 /// void dispose() {
@@ -416,27 +398,20 @@ class ScaleTransition extends AnimatedWidget {
 /// animated by a [CurvedAnimation] set to [Curves.elasticOut]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/rotation_transition.mp4}
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// The following code implements the [RotationTransition] as seen in the video
 /// above:
 ///
 /// ```dart
-/// AnimationController _controller;
-/// Animation<double> _animation;
-///
-/// @override
-/// void initState() {
-///   super.initState();
-///   _controller = AnimationController(
-///     duration: const Duration(seconds: 2),
-///     vsync: this,
-///   )..repeat(reverse: true);
-///   _animation = CurvedAnimation(
-///     parent: _controller,
-///     curve: Curves.elasticOut,
-///   );
-/// }
+/// late AnimationController _controller = AnimationController(
+///   duration: const Duration(seconds: 2),
+///   vsync: this,
+/// )..repeat(reverse: true);
+/// late Animation<double> _animation = CurvedAnimation(
+///   parent: _controller,
+///   curve: Curves.elasticOut,
+/// );
 ///
 /// @override
 /// void dispose() {
@@ -525,28 +500,21 @@ class RotationTransition extends AnimatedWidget {
 /// animated by a [CurvedAnimation] set to [Curves.fastOutSlowIn]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/size_transition.mp4}
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// This code defines a widget that uses [SizeTransition] to change the size
 /// of [FlutterLogo] continually. It is built with a [Scaffold]
 /// where the internal widget has space to change its size.
 ///
 /// ```dart
-/// AnimationController _controller;
-/// Animation<double> _animation;
-///
-/// @override
-/// void initState() {
-///   super.initState();
-///   _controller = AnimationController(
-///     duration: const Duration(seconds: 3),
-///     vsync: this,
-///   )..repeat();
-///   _animation = CurvedAnimation(
-///     parent: _controller,
-///     curve: Curves.fastOutSlowIn,
-///   );
-/// }
+/// late AnimationController _controller = AnimationController(
+///   duration: const Duration(seconds: 3),
+///   vsync: this,
+/// )..repeat();
+/// late Animation<double> _animation = CurvedAnimation(
+///   parent: _controller,
+///   curve: Curves.fastOutSlowIn,
+/// );
 ///
 /// @override
 /// void dispose() {
@@ -658,27 +626,20 @@ class SizeTransition extends AnimatedWidget {
 /// Here's an illustration of the [FadeTransition] widget, with it's [opacity]
 /// animated by a [CurvedAnimation] set to [Curves.fastOutSlowIn]:
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// The following code implements the [FadeTransition] using
 /// the Flutter logo:
 ///
 /// ```dart
-/// AnimationController _controller;
-/// Animation<double> _animation;
-///
-/// @override
-/// void initState() {
-///   super.initState();
-///   _controller = AnimationController(
-///     duration: const Duration(seconds: 2),
-///     vsync: this,
-///   )..repeat(reverse: true);
-///   _animation = CurvedAnimation(
-///     parent: _controller,
-///     curve: Curves.easeIn,
-///   );
-/// }
+/// late AnimationController _controller = AnimationController(
+///   duration: const Duration(seconds: 2),
+///   vsync: this,
+/// )..repeat(reverse: true);
+/// late Animation<double> _animation = CurvedAnimation(
+///   parent: _controller,
+///   curve: Curves.easeIn,
+/// );
 ///
 /// @override
 /// void dispose() {
@@ -762,21 +723,23 @@ class FadeTransition extends SingleChildRenderObjectWidget {
 
 /// Animates the opacity of a sliver widget.
 ///
-/// {@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] that uses a
 /// [SliverFadeTransition] to fade the list in and out.
 ///
 /// ```dart
 /// class _MyStatefulWidgetState extends State<MyStatefulWidget> with SingleTickerProviderStateMixin {
-///   AnimationController controller;
-///   Animation<double> animation;
+///   late AnimationController controller = AnimationController(
+///     duration: const Duration(milliseconds: 1000),
+///     vsync: this,
+///   );
+///   late Animation<double> animation = CurvedAnimation(
+///     parent: controller,
+///     curve: Curves.easeIn,
+///   );
 ///
 ///   initState() {
 ///     super.initState();
-///     controller = AnimationController(
-///         duration: const Duration(milliseconds: 1000), vsync: this);
-///     animation = CurvedAnimation(parent: controller, curve: Curves.easeIn);
-///
 ///     animation.addStatusListener((status) {
 ///       if (status == AnimationStatus.completed) {
 ///         controller.reverse();
@@ -904,22 +867,16 @@ class RelativeRectTween extends Tween<RelativeRect> {
 /// animated by a [CurvedAnimation] set to [Curves.elasticInOut]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/positioned_transition.mp4}
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// The following code implements the [PositionedTransition] as seen in the video
 /// above:
 ///
 /// ```dart
-/// AnimationController _controller;
-///
-/// @override
-/// void initState() {
-///   super.initState();
-///   _controller = AnimationController(
-///     duration: const Duration(seconds: 2),
-///     vsync: this,
-///   )..repeat(reverse: true);
-/// }
+/// late AnimationController _controller = AnimationController(
+///   duration: const Duration(seconds: 2),
+///   vsync: this,
+/// )..repeat(reverse: true);
 ///
 /// @override
 /// void dispose() {
@@ -1010,22 +967,16 @@ class PositionedTransition extends AnimatedWidget {
 /// animated by a [CurvedAnimation] set to [Curves.elasticInOut]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/relative_positioned_transition.mp4}
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// The following code implements the [RelativePositionedTransition] as seen in the video
 /// above:
 ///
 /// ```dart
-/// AnimationController _controller;
-///
-/// @override
-/// void initState() {
-///   super.initState();
-///   _controller = AnimationController(
-///     duration: const Duration(seconds: 2),
-///     vsync: this,
-///   )..repeat(reverse: true);
-/// }
+/// late AnimationController _controller = AnimationController(
+///   duration: const Duration(seconds: 2),
+///   vsync: this,
+/// )..repeat(reverse: true);
 ///
 /// @override
 /// void dispose() {
@@ -1051,7 +1002,7 @@ class PositionedTransition extends AnimatedWidget {
 ///             ).animate(CurvedAnimation(
 ///               parent: _controller,
 ///               curve: Curves.elasticInOut,
-///             )),
+///             )) as Animation<Rect>,
 ///             child: Padding(
 ///               padding: const EdgeInsets.all(8),
 ///               child: FlutterLogo()
@@ -1130,7 +1081,7 @@ class RelativePositionedTransition extends AnimatedWidget {
 /// [decoration] animated by a [CurvedAnimation] set to [Curves.decelerate]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/decorated_box_transition.mp4}
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 /// The following code implements the [DecoratedBoxTransition] as seen in the video
 /// above:
 ///
@@ -1160,16 +1111,10 @@ class RelativePositionedTransition extends AnimatedWidget {
 ///   ),
 /// );
 ///
-/// AnimationController _controller;
-///
-/// @override
-/// void initState() {
-///   _controller = AnimationController(
-///     vsync: this,
-///     duration: const Duration(seconds: 3),
-///   )..repeat(reverse: true);
-///   super.initState();
-/// }
+/// late AnimationController _controller = AnimationController(
+///   vsync: this,
+///   duration: const Duration(seconds: 3),
+/// )..repeat(reverse: true);
 ///
 /// @override
 /// void dispose() {
@@ -1310,15 +1255,15 @@ class AlignTransition extends AnimatedWidget {
 /// Animated version of a [DefaultTextStyle] that animates the different properties
 /// of its [TextStyle].
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// The following code implements the [DefaultTextStyleTransition] that shows
 /// a transition between thick blue font and thin red font.
 ///
 /// ```dart
-/// AnimationController _controller;
-/// TextStyleTween _styleTween;
-/// CurvedAnimation _curvedAnimation;
+/// late AnimationController _controller;
+/// late TextStyleTween _styleTween;
+/// late CurvedAnimation _curvedAnimation;
 ///
 /// @override
 /// void initState() {
@@ -1438,7 +1383,7 @@ class DefaultTextStyleTransition extends AnimatedWidget {
 /// Using this pre-built child is entirely optional, but can improve
 /// performance significantly in some cases and is therefore a good practice.
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// This code defines a widget that spins a green square continually. It is
 /// built with an [AnimatedBuilder] and makes use of the [child] feature to
@@ -1449,16 +1394,10 @@ class DefaultTextStyleTransition extends AnimatedWidget {
 /// ```
 ///
 /// ```dart
-/// AnimationController _controller;
-///
-/// @override
-/// void initState() {
-///   super.initState();
-///   _controller = AnimationController(
-///     duration: const Duration(seconds: 10),
-///     vsync: this,
-///   )..repeat();
-/// }
+/// late AnimationController _controller = AnimationController(
+///   duration: const Duration(seconds: 10),
+///   vsync: this,
+/// )..repeat();
 ///
 /// @override
 /// void dispose() {
@@ -1478,7 +1417,7 @@ class DefaultTextStyleTransition extends AnimatedWidget {
 ///         child: Text('Whee!'),
 ///       ),
 ///     ),
-///     builder: (BuildContext context, Widget child) {
+///     builder: (BuildContext context, Widget? child) {
 ///       return Transform.rotate(
 ///         angle: _controller.value * 2.0 * math.pi,
 ///         child: child,
diff --git a/packages/flutter/lib/src/widgets/tween_animation_builder.dart b/packages/flutter/lib/src/widgets/tween_animation_builder.dart
index 66eb55a8be..48f8d38b7a 100644
--- a/packages/flutter/lib/src/widgets/tween_animation_builder.dart
+++ b/packages/flutter/lib/src/widgets/tween_animation_builder.dart
@@ -61,7 +61,7 @@ import 'value_listenable_builder.dart';
 ///
 /// ## Example Code
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center}
 /// This example shows an [IconButton] that "zooms" in when the widget first
 /// builds (its size smoothly increases from 0 to 24) and whenever the button
 /// is pressed, it smoothly changes its size to the new target value of either
@@ -75,11 +75,11 @@ import 'value_listenable_builder.dart';
 ///   return TweenAnimationBuilder(
 ///     tween: Tween<double>(begin: 0, end: targetValue),
 ///     duration: Duration(seconds: 1),
-///     builder: (BuildContext context, double size, Widget child) {
+///     builder: (BuildContext context, double size, Widget? child) {
 ///       return IconButton(
 ///         iconSize: size,
 ///         color: Colors.blue,
-///         icon: child,
+///         icon: child!,
 ///         onPressed: () {
 ///           setState(() {
 ///             targetValue = targetValue == 24.0 ? 48.0 : 24.0;