[NNBD] Migrate sample code pt 4 (#72842)

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() {

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

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.
 ///               }
 ///             },

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

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(

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;