Migrate more sample code to nnbd (#72829)

packages/flutter/lib/src/animation/curves.dart

@@ -319,7 +319,7 @@ class Cubic extends Curve {
 /// part of the curve, or hardly at all in another part of the curve, depending
 /// on the definition of the curve.
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 /// This example shows how to use a [Curve2D] to modify the position of a widget
 /// so that it can follow an arbitrary path.
 ///
@@ -346,16 +346,12 @@ class Cubic extends Curve {
 ///
 /// class FollowCurve2D extends StatefulWidget {
 ///   const FollowCurve2D({
-///     Key key,
-///     @required this.path,
+///     Key? key,
+///     required this.path,
 ///     this.curve = Curves.easeInOut,
-///     @required this.child,
+///     required this.child,
 ///     this.duration = const Duration(seconds: 1),
-///   })  : assert(path != null),
-///         assert(curve != null),
-///         assert(child != null),
-///         assert(duration != null),
-///         super(key: key);
+///   }) : super(key: key);
 ///
 ///   final Curve2D path;
 ///   final Curve curve;
@@ -368,9 +364,9 @@ class Cubic extends Curve {
 ///
 /// class _FollowCurve2DState extends State<FollowCurve2D> with TickerProviderStateMixin {
 ///   // The animation controller for this animation.
-///   AnimationController controller;
+///   late AnimationController controller;
 ///   // The animation that will be used to apply the widget's animation curve.
-///   Animation<double> animation;
+///   late Animation<double> animation;
 ///
 ///   @override
 ///   void initState() {
@@ -414,7 +410,7 @@ class Cubic extends Curve {
 ///         child: CircleAvatar(
 ///           backgroundColor: Colors.yellow,
 ///           child: DefaultTextStyle(
-///             style: Theme.of(context).textTheme.headline6,
+///             style: Theme.of(context).textTheme.headline6!,
 ///             child: Text("B"), // Buzz, buzz!
 ///           ),
 ///         ),

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

@@ -266,16 +266,12 @@ typedef RefreshCallback = Future<void> Function();
 /// sliver such as [CupertinoSliverNavigationBar] and your main scrollable
 /// content's sliver.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_cupertino}
 ///
 /// When the user scrolls past [refreshTriggerPullDistance],
 /// this sample shows the default iOS pull to refresh indicator for 1 second and
 /// adds a new item to the top of the list view.
 ///
-/// ```dart imports
-/// import 'package:flutter/cupertino.dart';
-/// ```
-///
 /// ```dart
 /// List<Color> colors = [
 ///   CupertinoColors.systemYellow,

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

@@ -30,9 +30,6 @@ import 'widget_inspector.dart';
 
 export 'dart:ui' show Locale;
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// The signature of [WidgetsApp.localeListResolutionCallback].
 ///
 /// A [LocaleListResolutionCallback] is responsible for computing the locale of the app's
@@ -883,7 +880,7 @@ class WidgetsApp extends StatefulWidget {
   ///       LogicalKeySet(LogicalKeyboardKey.select): const ActivateIntent(),
   ///     },
   ///     color: const Color(0xFFFF0000),
-  ///     builder: (BuildContext context, Widget child) {
+  ///     builder: (BuildContext context, Widget? child) {
   ///       return const Placeholder();
   ///     },
   ///   );
@@ -937,7 +934,7 @@ class WidgetsApp extends StatefulWidget {
   ///       ),
   ///     },
   ///     color: const Color(0xFFFF0000),
-  ///     builder: (BuildContext context, Widget child) {
+  ///     builder: (BuildContext context, Widget? child) {
   ///       return const Placeholder();
   ///     },
   ///   );

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

@@ -21,9 +21,6 @@ import 'widget_inspector.dart';
 
 export 'dart:ui' show AppLifecycleState, Locale;
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// Interface for classes that register with the Widgets layer binding.
 ///
 /// When used as a mixin, provides no-op method implementations.
@@ -43,7 +40,7 @@ export 'dart:ui' show AppLifecycleState, Locale;
 ///
 /// ```dart
 /// class AppLifecycleReactor extends StatefulWidget {
-///   const AppLifecycleReactor({ Key key }) : super(key: key);
+///   const AppLifecycleReactor({ Key? key }) : super(key: key);
 ///
 ///   @override
 ///   _AppLifecycleReactorState createState() => _AppLifecycleReactorState();
@@ -53,16 +50,16 @@ export 'dart:ui' show AppLifecycleState, Locale;
 ///   @override
 ///   void initState() {
 ///     super.initState();
-///     WidgetsBinding.instance.addObserver(this);
+///     WidgetsBinding.instance!.addObserver(this);
 ///   }
 ///
 ///   @override
 ///   void dispose() {
-///     WidgetsBinding.instance.removeObserver(this);
+///     WidgetsBinding.instance!.removeObserver(this);
 ///     super.dispose();
 ///   }
 ///
-///   AppLifecycleState _notification;
+///   late AppLifecycleState _notification;
 ///
 ///   @override
 ///   void didChangeAppLifecycleState(AppLifecycleState state) {
@@ -138,31 +135,31 @@ abstract class WidgetsBindingObserver {
   ///
   /// ```dart
   /// class MetricsReactor extends StatefulWidget {
-  ///   const MetricsReactor({ Key key }) : super(key: key);
+  ///   const MetricsReactor({ Key? key }) : super(key: key);
   ///
   ///   @override
   ///   _MetricsReactorState createState() => _MetricsReactorState();
   /// }
   ///
   /// class _MetricsReactorState extends State<MetricsReactor> with WidgetsBindingObserver {
-  ///   Size _lastSize;
+  ///   late Size _lastSize;
   ///
   ///   @override
   ///   void initState() {
   ///     super.initState();
-  ///     _lastSize = WidgetsBinding.instance.window.physicalSize;
-  ///     WidgetsBinding.instance.addObserver(this);
+  ///     _lastSize = WidgetsBinding.instance!.window.physicalSize;
+  ///     WidgetsBinding.instance!.addObserver(this);
   ///   }
   ///
   ///   @override
   ///   void dispose() {
-  ///     WidgetsBinding.instance.removeObserver(this);
+  ///     WidgetsBinding.instance!.removeObserver(this);
   ///     super.dispose();
   ///   }
   ///
   ///   @override
   ///   void didChangeMetrics() {
-  ///     setState(() { _lastSize = WidgetsBinding.instance.window.physicalSize; });
+  ///     setState(() { _lastSize = WidgetsBinding.instance!.window.physicalSize; });
   ///   }
   ///
   ///   @override
@@ -196,7 +193,7 @@ abstract class WidgetsBindingObserver {
   ///
   /// ```dart
   /// class TextScaleFactorReactor extends StatefulWidget {
-  ///   const TextScaleFactorReactor({ Key key }) : super(key: key);
+  ///   const TextScaleFactorReactor({ Key? key }) : super(key: key);
   ///
   ///   @override
   ///   _TextScaleFactorReactorState createState() => _TextScaleFactorReactorState();
@@ -206,20 +203,20 @@ abstract class WidgetsBindingObserver {
   ///   @override
   ///   void initState() {
   ///     super.initState();
-  ///     WidgetsBinding.instance.addObserver(this);
+  ///     WidgetsBinding.instance!.addObserver(this);
   ///   }
   ///
   ///   @override
   ///   void dispose() {
-  ///     WidgetsBinding.instance.removeObserver(this);
+  ///     WidgetsBinding.instance!.removeObserver(this);
   ///     super.dispose();
   ///   }
   ///
-  ///   double _lastTextScaleFactor;
+  ///   late double _lastTextScaleFactor;
   ///
   ///   @override
   ///   void didChangeTextScaleFactor() {
-  ///     setState(() { _lastTextScaleFactor = WidgetsBinding.instance.window.textScaleFactor; });
+  ///     setState(() { _lastTextScaleFactor = WidgetsBinding.instance!.window.textScaleFactor; });
   ///   }
   ///
   ///   @override

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

@@ -23,8 +23,7 @@ import 'sliver.dart';
 import 'viewport.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// int itemCount;
+// late int itemCount;
 
 /// A representation of how a [ScrollView] should dismiss the on-screen
 /// keyboard.
@@ -468,7 +467,7 @@ abstract class ScrollView extends StatelessWidget {
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 ///
 /// By default, if items are inserted at the "top" of a scrolling container like
 /// [ListView] or [CustomScrollView], the top item and all of the items below it
@@ -1335,7 +1334,7 @@ class ListView extends BoxScrollView {
   ///             },
   ///             childCount: items.length,
   ///             findChildIndexCallback: (Key key) {
-  ///               final ValueKey valueKey = key;
+  ///               final ValueKey valueKey = key as ValueKey;
   ///               final String data = valueKey.value;
   ///               return items.indexOf(data);
   ///             }
@@ -1358,7 +1357,10 @@ class ListView extends BoxScrollView {
   /// }
   ///
   /// class KeepAlive extends StatefulWidget {
-  ///   const KeepAlive({Key key, this.data}) : super(key: key);
+  ///   const KeepAlive({
+  ///     required Key key,
+  ///     required this.data,
+  ///   }) : super(key: key);
   ///
   ///   final String data;
   ///

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

@@ -17,8 +17,7 @@ export 'package:flutter/rendering.dart' show
   SliverGridDelegateWithMaxCrossAxisExtent;
 
 // Examples can assume:
-// // @dart = 2.9
-// SliverGridDelegateWithMaxCrossAxisExtent _gridDelegate;
+// late SliverGridDelegateWithMaxCrossAxisExtent _gridDelegate;
 
 /// A callback which produces a semantic index given a widget and the local index.
 ///

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

@@ -29,7 +29,7 @@ import 'routes.dart';
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// ```dart
 /// bool shouldPop = true;
 /// @override