Migrate more doc samples (#72392)

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

@@ -11,8 +11,7 @@ import 'framework.dart';
 import 'image.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// BuildContext context;
+// late BuildContext context;
 
 /// A widget that paints a [Decoration] either before or after its child paints.
 ///
@@ -219,7 +218,7 @@ class DecoratedBox extends SingleChildRenderObjectWidget {
 /// ```dart
 /// Container(
 ///   constraints: BoxConstraints.expand(
-///     height: Theme.of(context).textTheme.headline4.fontSize * 1.1 + 200.0,
+///     height: Theme.of(context).textTheme.headline4!.fontSize! * 1.1 + 200.0,
 ///   ),
 ///   padding: const EdgeInsets.all(8.0),
 ///   color: Colors.blue[600],
@@ -227,7 +226,7 @@ class DecoratedBox extends SingleChildRenderObjectWidget {
 ///   child: Text('Hello World',
 ///     style: Theme.of(context)
 ///         .textTheme
-///         .headline4
+///         .headline4!
 ///         .copyWith(color: Colors.white)),
 ///   transform: Matrix4.rotationZ(0.1),
 /// )

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

@@ -14,8 +14,7 @@ import 'implicit_animations.dart';
 import 'transitions.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// Uint8List bytes;
+// late Uint8List bytes;
 
 /// An image that shows a [placeholder] image while the target [image] is
 /// loading, then fades in the new image when it loads.

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

@@ -30,12 +30,11 @@ export 'package:flutter/foundation.dart' show Key, LocalKey, ValueKey;
 export 'package:flutter/rendering.dart' show RenderObject, RenderBox, debugDumpRenderTree, debugDumpLayerTree;
 
 // Examples can assume:
-// // @dart = 2.9
-// BuildContext context;
+// late BuildContext context;
 // void setState(VoidCallback fn) { }
 // abstract class RenderFrogJar extends RenderObject { }
 // abstract class FrogJar extends RenderObjectWidget { }
-// abstract class FrogJarParentData extends ParentData { Size size; }
+// abstract class FrogJarParentData extends ParentData { late Size size; }
 
 // KEYS
 
@@ -624,7 +623,7 @@ abstract class Widget extends DiagnosticableTree {
 ///
 /// ```dart
 /// class GreenFrog extends StatelessWidget {
-///   const GreenFrog({ Key key }) : super(key: key);
+///   const GreenFrog({ Key? key }) : super(key: key);
 ///
 ///   @override
 ///   Widget build(BuildContext context) {
@@ -642,13 +641,13 @@ abstract class Widget extends DiagnosticableTree {
 /// ```dart
 /// class Frog extends StatelessWidget {
 ///   const Frog({
-///     Key key,
+///     Key? key,
 ///     this.color = const Color(0xFF2DBD3A),
 ///     this.child,
 ///   }) : super(key: key);
 ///
 ///   final Color color;
-///   final Widget child;
+///   final Widget? child;
 ///
 ///   @override
 ///   Widget build(BuildContext context) {
@@ -841,7 +840,7 @@ abstract class StatelessWidget extends Widget {
 ///
 /// ```dart
 /// class YellowBird extends StatefulWidget {
-///   const YellowBird({ Key key }) : super(key: key);
+///   const YellowBird({ Key? key }) : super(key: key);
 ///
 ///   @override
 ///   _YellowBirdState createState() => _YellowBirdState();
@@ -864,13 +863,13 @@ abstract class StatelessWidget extends Widget {
 /// ```dart
 /// class Bird extends StatefulWidget {
 ///   const Bird({
-///     Key key,
+///     Key? key,
 ///     this.color = const Color(0xFFFFE306),
 ///     this.child,
 ///   }) : super(key: key);
 ///
 ///   final Color color;
-///   final Widget child;
+///   final Widget? child;
 ///
 ///   _BirdState createState() => _BirdState();
 /// }
@@ -1531,21 +1530,19 @@ abstract class ProxyWidget extends Widget {
 /// ```dart
 /// class FrogSize extends ParentDataWidget<FrogJarParentData> {
 ///   FrogSize({
-///     Key key,
-///     @required this.size,
-///     @required Widget child,
-///   }) : assert(child != null),
-///        assert(size != null),
-///        super(key: key, child: child);
+///     Key? key,
+///     required this.size,
+///     required Widget child,
+///   }) : super(key: key, child: child);
 ///
 ///   final Size size;
 ///
 ///   @override
 ///   void applyParentData(RenderObject renderObject) {
-///     final FrogJarParentData parentData = renderObject.parentData;
+///     final FrogJarParentData parentData = renderObject.parentData! as FrogJarParentData;
 ///     if (parentData.size != size) {
 ///       parentData.size = size;
-///       final RenderFrogJar targetParent = renderObject.parent;
+///       final RenderFrogJar targetParent = renderObject.parent! as RenderFrogJar;
 ///       targetParent.markNeedsLayout();
 ///     }
 ///   }
@@ -1684,17 +1681,17 @@ abstract class ParentDataWidget<T extends ParentData> extends ProxyWidget {
 /// ```dart
 /// class FrogColor extends InheritedWidget {
 ///   const FrogColor({
-///     Key key,
-///     @required this.color,
-///     @required Widget child,
-///   }) : assert(color != null),
-///        assert(child != null),
-///        super(key: key, child: child);
+///     Key? key,
+///     required this.color,
+///     required Widget child,
+///   }) : super(key: key, child: child);
 ///
 ///   final Color color;
 ///
 ///   static FrogColor of(BuildContext context) {
-///     return context.dependOnInheritedWidgetOfExactType<FrogColor>();
+///     final FrogColor? result = context.dependOnInheritedWidgetOfExactType<FrogColor>();
+///     assert(result != null, 'No FrogColor found in context');
+///     return result!;
 ///   }
 ///
 ///   @override
@@ -2420,7 +2417,7 @@ abstract class BuildContext {
   /// {@tool snippet}
   ///
   /// ```dart
-  /// ScrollableState scrollable = context.findAncestorStateOfType<ScrollableState>();
+  /// ScrollableState? scrollable = context.findAncestorStateOfType<ScrollableState>();
   /// ```
   /// {@end-tool}
   T? findAncestorStateOfType<T extends State>();
@@ -4436,7 +4433,7 @@ typedef ErrorWidgetBuilder = Widget Function(FlutterErrorDetails details);
 ///
 /// It is possible to override this widget.
 ///
-/// {@tool sample --template=freeform_no_null_safety}
+/// {@tool sample --template=freeform}
 /// ```dart
 /// import 'package:flutter/material.dart';
 ///

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

@@ -27,9 +27,6 @@ import 'sliver.dart';
 import 'sliver_fill.dart';
 import 'viewport.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// A controller for [PageView].
 ///
 /// A page controller lets you manipulate which page is visible in a [PageView].
@@ -49,19 +46,13 @@ import 'viewport.dart';
 ///
 /// ```dart
 /// class MyPageView extends StatefulWidget {
-///   MyPageView({Key key}) : super(key: key);
+///   MyPageView({Key? key}) : super(key: key);
 ///
 ///   _MyPageViewState createState() => _MyPageViewState();
 /// }
 ///
 /// class _MyPageViewState extends State<MyPageView> {
-///   PageController _pageController;
-///
-///   @override
-///   void initState() {
-///     super.initState();
-///     _pageController = PageController();
-///   }
+///   PageController _pageController = PageController();
 ///
 ///   @override
 ///   void dispose() {
@@ -725,7 +716,7 @@ class PageView extends StatefulWidget {
   ///             },
   ///             childCount: items.length,
   ///             findChildIndexCallback: (Key key) {
-  ///               final ValueKey valueKey = key;
+  ///               final ValueKey<String> valueKey = key as ValueKey<String>;
   ///               final String data = valueKey.value;
   ///               return items.indexOf(data);
   ///             }
@@ -748,7 +739,7 @@ class PageView extends StatefulWidget {
   /// }
   ///
   /// class KeepAlive extends StatefulWidget {
-  ///   const KeepAlive({Key key, this.data}) : super(key: key);
+  ///   const KeepAlive({Key? key, required this.data}) : super(key: key);
   ///
   ///   final String data;
   ///

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

@@ -17,12 +17,15 @@ import 'scroll_simulation.dart';
 export 'package:flutter/physics.dart' show Simulation, ScrollSpringSimulation, Tolerance;
 
 // Examples can assume:
-// // @dart = 2.9
 // class FooScrollPhysics extends ScrollPhysics {
-//   const FooScrollPhysics({ ScrollPhysics parent }): super(parent: parent);
+//   const FooScrollPhysics({ ScrollPhysics? parent }): super(parent: parent);
+//   @override
+//   FooScrollPhysics applyTo(ScrollPhysics? ancestor) {
+//     return FooScrollPhysics(parent: buildParent(ancestor));
+//   }
 // }
 // class BarScrollPhysics extends ScrollPhysics {
-//   const BarScrollPhysics({ ScrollPhysics parent }): super(parent: parent);
+//   const BarScrollPhysics({ ScrollPhysics? parent }): super(parent: parent);
 // }
 
 /// Determines the physics of a [Scrollable] widget.

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

@@ -6,9 +6,6 @@ import 'package:flutter/foundation.dart';
 
 import 'framework.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// Builds a [Widget] when given a concrete value of a [ValueListenable<T>].
 ///
 /// If the `child` parameter provided to the [ValueListenableBuilder] is not
@@ -50,7 +47,7 @@ typedef ValueWidgetBuilder<T> = Widget Function(BuildContext context, T value, W
 ///
 /// ```dart
 /// class MyHomePage extends StatefulWidget {
-///   MyHomePage({Key key, this.title}) : super(key: key);
+///   MyHomePage({Key? key, required this.title}) : super(key: key);
 ///   final String title;
 ///
 ///   @override
@@ -72,14 +69,14 @@ typedef ValueWidgetBuilder<T> = Widget Function(BuildContext context, T value, W
 ///           children: <Widget>[
 ///             Text('You have pushed the button this many times:'),
 ///             ValueListenableBuilder(
-///               builder: (BuildContext context, int value, Widget child) {
+///               builder: (BuildContext context, int value, Widget? child) {
 ///                 // This builder will only get called when the _counter
 ///                 // is updated.
 ///                 return Row(
 ///                   mainAxisAlignment: MainAxisAlignment.spaceEvenly,
 ///                   children: <Widget>[
 ///                     Text('$value'),
-///                     child,
+///                     child!,
 ///                   ],
 ///                 );
 ///               },