diff --git a/packages/flutter/lib/src/material/circle_avatar.dart b/packages/flutter/lib/src/material/circle_avatar.dart
index 5d5020d128..3ff42459a1 100644
--- a/packages/flutter/lib/src/material/circle_avatar.dart
+++ b/packages/flutter/lib/src/material/circle_avatar.dart
@@ -9,8 +9,7 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// String userAvatarUrl;
+// late String userAvatarUrl;
 
 /// A circle that represents a user.
 ///
diff --git a/packages/flutter/lib/src/material/colors.dart b/packages/flutter/lib/src/material/colors.dart
index 6e01c88c2c..240f34ebb8 100644
--- a/packages/flutter/lib/src/material/colors.dart
+++ b/packages/flutter/lib/src/material/colors.dart
@@ -6,9 +6,6 @@ import 'dart:ui' show Color;
 
 import 'package:flutter/painting.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// Defines a single color as well a color swatch with ten shades of the color.
 ///
 /// The color's shades are referred to by index. The greater the index, the
@@ -113,7 +110,7 @@ class MaterialAccentColor extends ColorSwatch<int> {
 /// using an integer for the specific color desired, as follows:
 ///
 /// ```dart
-/// Color selection = Colors.green[400]; // Selects a mid-range green.
+/// Color selection = Colors.green[400]!; // Selects a mid-range green.
 /// ```
 /// {@end-tool}
 /// {@tool snippet}
diff --git a/packages/flutter/lib/src/material/dialog.dart b/packages/flutter/lib/src/material/dialog.dart
index 4012977f2b..c9d406a832 100644
--- a/packages/flutter/lib/src/material/dialog.dart
+++ b/packages/flutter/lib/src/material/dialog.dart
@@ -18,9 +18,8 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 // Examples can assume:
-// // @dart = 2.9
 // enum Department { treasury, state }
-// BuildContext context;
+// late BuildContext context;
 
 const EdgeInsets _defaultInsetPadding = EdgeInsets.symmetric(horizontal: 40.0, vertical: 24.0);
 
@@ -706,6 +705,9 @@ class SimpleDialogOption extends StatelessWidget {
 ///     case Department.state:
 ///       // ...
 ///     break;
+///     case null:
+///       // dialog dismissed
+///     break;
 ///   }
 /// }
 /// ```
diff --git a/packages/flutter/lib/src/material/navigation_rail.dart b/packages/flutter/lib/src/material/navigation_rail.dart
index abf939c590..d5f12f76f1 100644
--- a/packages/flutter/lib/src/material/navigation_rail.dart
+++ b/packages/flutter/lib/src/material/navigation_rail.dart
@@ -16,9 +16,6 @@ import 'navigation_rail_theme.dart';
 import 'theme.dart';
 import 'theme_data.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// A material widget that is meant to be displayed at the left or right of an
 /// app to navigate between a small number of views, typically between three and
 /// five.
@@ -41,7 +38,7 @@ import 'theme_data.dart';
 /// [https://github.com/flutter/samples/blob/master/experimental/web_dashboard/lib/src/widgets/third_party/adaptive_scaffold.dart]
 /// for an example.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 ///
 /// This example shows a [NavigationRail] used within a Scaffold with 3
 /// [NavigationRailDestination]s. The main content is separated by a divider
@@ -350,12 +347,12 @@ class NavigationRail extends StatefulWidget {
   ///   final Animation<double> animation = NavigationRail.extendedAnimation(context);
   ///   return AnimatedBuilder(
   ///     animation: animation,
-  ///     builder: (BuildContext context, Widget child) {
+  ///     builder: (BuildContext context, Widget? child) {
   ///       // The extended fab has a shorter height than the regular fab.
   ///       return Container(
   ///         height: 56,
   ///         padding: EdgeInsets.symmetric(
-  ///           vertical: lerpDouble(0, 6, animation.value),
+  ///           vertical: lerpDouble(0, 6, animation.value)!,
   ///         ),
   ///         child: animation.value == 0
   ///           ? FloatingActionButton(
diff --git a/packages/flutter/lib/src/material/reorderable_list.dart b/packages/flutter/lib/src/material/reorderable_list.dart
index 45a0a261f5..8e3ea83392 100644
--- a/packages/flutter/lib/src/material/reorderable_list.dart
+++ b/packages/flutter/lib/src/material/reorderable_list.dart
@@ -12,7 +12,6 @@ import 'material.dart';
 import 'material_localizations.dart';
 
 // Examples can assume:
-// // @dart = 2.9
 // class MyDataObject { }
 
 /// The callback used by [ReorderableListView] to move an item to a new
@@ -60,7 +59,7 @@ typedef ReorderCallback = void Function(int oldIndex, int newIndex);
 /// The [onReorder] parameter is required and will be called when a child
 /// widget is dragged to a new position.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 ///
 /// ```dart
 /// List<String> _list = List.generate(5, (i) => "${i}");
diff --git a/packages/flutter/lib/src/material/scaffold.dart b/packages/flutter/lib/src/material/scaffold.dart
index 2d428f6e2c..e0560cee6f 100644
--- a/packages/flutter/lib/src/material/scaffold.dart
+++ b/packages/flutter/lib/src/material/scaffold.dart
@@ -30,12 +30,11 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// TabController tabController;
+// late TabController tabController;
 // void setState(VoidCallback fn) { }
-// String appBarTitle;
-// int tabCount;
-// TickerProvider tickerProvider;
+// late String appBarTitle;
+// late int tabCount;
+// late TickerProvider tickerProvider;
 
 const FloatingActionButtonLocation _kDefaultFloatingActionButtonLocation = FloatingActionButtonLocation.endFloat;
 const FloatingActionButtonAnimator _kDefaultFloatingActionButtonAnimator = FloatingActionButtonAnimator.scaling;
@@ -69,7 +68,7 @@ enum _ScaffoldSlot {
 /// [BuildContext] via [ScaffoldMessenger.of] and use the
 /// [ScaffoldMessengerState.showSnackBar] function.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold_center}
 ///
 /// Here is an example of showing a [SnackBar] when the user presses a button.
 ///
@@ -112,7 +111,7 @@ class ScaffoldMessenger extends StatefulWidget {
   /// The state from the closest instance of this class that encloses the given
   /// context.
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_scaffold_center}
   /// Typical usage of the [ScaffoldMessenger.of] function is to call it in
   /// response to a user gesture or an application state change.
   ///
@@ -138,7 +137,7 @@ class ScaffoldMessenger extends StatefulWidget {
   /// function. The [MaterialApp.scaffoldMessengerKey] refers to the root
   /// ScaffoldMessenger that is provided by default.
   ///
-  /// {@tool dartpad --template=freeform_no_null_safety}
+  /// {@tool dartpad --template=freeform}
   /// Sometimes [SnackBar]s are produced by code that doesn't have ready access
   /// to a valid [BuildContext]. One such example of this is when you show a
   /// SnackBar from a method outside of the `build` function. In these
@@ -166,7 +165,7 @@ class ScaffoldMessenger extends StatefulWidget {
   ///       _counter++;
   ///     });
   ///     if (_counter % 10 == 0) {
-  ///       _scaffoldMessengerKey.currentState.showSnackBar(const SnackBar(
+  ///       _scaffoldMessengerKey.currentState!.showSnackBar(const SnackBar(
   ///         content: Text('A multiple of ten!'),
   ///       ));
   ///     }
@@ -304,7 +303,7 @@ class ScaffoldMessengerState extends State<ScaffoldMessenger> with TickerProvide
   /// See [ScaffoldMessenger.of] for information about how to obtain the
   /// [ScaffoldMessengerState].
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_scaffold_center}
   ///
   /// Here is an example of showing a [SnackBar] when the user presses a button.
   ///
@@ -1259,7 +1258,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// [ScaffoldState] for the current [BuildContext] via [Scaffold.of] and use the
 /// [ScaffoldState.showBottomSheet] function.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// This example shows a [Scaffold] with a [body] and [FloatingActionButton].
 /// The [body] is a [Text] placed in a [Center] in order to center the text
 /// within the [Scaffold]. The [FloatingActionButton] is connected to a
@@ -1288,7 +1287,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// This example shows a [Scaffold] with a blueGrey [backgroundColor], [body]
 /// and [FloatingActionButton]. The [body] is a [Text] placed in a [Center] in
 /// order to center the text within the [Scaffold]. The [FloatingActionButton]
@@ -1318,7 +1317,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// This example shows a [Scaffold] with an [AppBar], a [BottomAppBar] and a
 /// [FloatingActionButton]. The [body] is a [Text] placed in a [Center] in order
 /// to center the text within the [Scaffold]. The [FloatingActionButton] is
@@ -1560,7 +1559,7 @@ class Scaffold extends StatefulWidget {
   ///
   /// To close the drawer, use [Navigator.pop].
   ///
-  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material}
   /// To disable the drawer edge swipe, set the
   /// [Scaffold.drawerEnableOpenDragGesture] to false. Then, use
   /// [ScaffoldState.openDrawer] to open the drawer and [Navigator.pop] to close
@@ -1570,7 +1569,7 @@ class Scaffold extends StatefulWidget {
   /// final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
   ///
   /// void _openDrawer() {
-  ///   _scaffoldKey.currentState.openDrawer();
+  ///   _scaffoldKey.currentState!.openDrawer();
   /// }
   ///
   /// void _closeDrawer() {
@@ -1623,7 +1622,7 @@ class Scaffold extends StatefulWidget {
   ///
   /// To close the drawer, use [Navigator.pop].
   ///
-  /// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material}
   /// To disable the drawer edge swipe, set the
   /// [Scaffold.endDrawerEnableOpenDragGesture] to false. Then, use
   /// [ScaffoldState.openEndDrawer] to open the drawer and [Navigator.pop] to
@@ -1633,7 +1632,7 @@ class Scaffold extends StatefulWidget {
   /// final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
   ///
   /// void _openEndDrawer() {
-  ///   _scaffoldKey.currentState.openEndDrawer();
+  ///   _scaffoldKey.currentState!.openEndDrawer();
   /// }
   ///
   /// void _closeEndDrawer() {
@@ -1790,7 +1789,7 @@ class Scaffold extends StatefulWidget {
   /// If no instance of this class encloses the given context, will cause an
   /// assert in debug mode, and throw an exception in release mode.
   ///
-  /// {@tool dartpad --template=freeform_no_null_safety}
+  /// {@tool dartpad --template=freeform}
   /// Typical usage of the [Scaffold.of] function is to call it from within the
   /// `build` method of a child of a [Scaffold].
   ///
@@ -1861,7 +1860,7 @@ class Scaffold extends StatefulWidget {
   /// ```
   /// {@end-tool}
   ///
-  /// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_material}
   /// When the [Scaffold] is actually created in the same `build` function, the
   /// `context` argument to the `build` function can't be used to find the
   /// [Scaffold] (since it's "above" the widget being returned in the widget
@@ -2173,7 +2172,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   /// See [ScaffoldMessenger.of] for information about how to obtain the
   /// [ScaffoldMessengerState].
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_scaffold_center}
   ///
   /// Here is an example of showing a [SnackBar] when the user presses a button.
   ///
@@ -2547,7 +2546,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   /// of the app. Modal bottom sheets can be created and displayed with the
   /// [showModalBottomSheet] function.
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_scaffold}
   ///
   /// This example demonstrates how to use `showBottomSheet` to display a
   /// bottom sheet when a user taps a button. It also demonstrates how to
diff --git a/packages/flutter/lib/src/material/tab_controller.dart b/packages/flutter/lib/src/material/tab_controller.dart
index 70fef1bc40..1b79613651 100644
--- a/packages/flutter/lib/src/material/tab_controller.dart
+++ b/packages/flutter/lib/src/material/tab_controller.dart
@@ -9,8 +9,7 @@ import 'package:flutter/widgets.dart';
 import 'constants.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// BuildContext context;
+// late BuildContext context;
 
 /// Coordinates tab selection between a [TabBar] and a [TabBarView].
 ///
@@ -33,7 +32,7 @@ import 'constants.dart';
 ///
 /// ```dart
 /// class MyTabbedPage extends StatefulWidget {
-///   const MyTabbedPage({ Key key }) : super(key: key);
+///   const MyTabbedPage({ Key? key }) : super(key: key);
 ///   @override
 ///   _MyTabbedPageState createState() => _MyTabbedPageState();
 /// }
@@ -44,7 +43,7 @@ import 'constants.dart';
 ///     Tab(text: 'RIGHT'),
 ///   ];
 ///
-///   TabController _tabController;
+///   late TabController _tabController;
 ///
 ///   @override
 ///   void initState() {
@@ -70,7 +69,7 @@ import 'constants.dart';
 ///       body: TabBarView(
 ///         controller: _tabController,
 ///         children: myTabs.map((Tab tab) {
-///           final String label = tab.text.toLowerCase();
+///           final String label = tab.text!.toLowerCase();
 ///           return Center(
 ///             child: Text(
 ///               'This is the $label tab',
@@ -85,7 +84,7 @@ import 'constants.dart';
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 ///
 /// This example shows how to listen to page updates in [TabBar] and [TabBarView]
 /// when using [DefaultTabController].
@@ -106,7 +105,7 @@ import 'constants.dart';
 ///     // closest DefaultTabController.
 ///     child: Builder(
 ///       builder: (BuildContext context) {
-///         final TabController tabController = DefaultTabController.of(context);
+///         final TabController tabController = DefaultTabController.of(context)!;
 ///         tabController.addListener(() {
 ///           if (!tabController.indexIsChanging) {
 ///             // Your code goes here.
@@ -123,7 +122,7 @@ import 'constants.dart';
 ///             children: tabs.map((Tab tab){
 ///               return Center(
 ///                 child: Text(
-///                   tab.text + ' Tab',
+///                   tab.text! + ' Tab',
 ///                   style: Theme.of(context).textTheme.headline5,
 ///                 ),
 ///               );
@@ -405,7 +404,7 @@ class DefaultTabController extends StatefulWidget {
   /// Typical usage is as follows:
   ///
   /// ```dart
-  /// TabController controller = DefaultTabController.of(context);
+  /// TabController controller = DefaultTabController.of(context)!;
   /// ```
   /// {@end-tool}
   static TabController? of(BuildContext context) {
diff --git a/packages/flutter/lib/src/material/text_field.dart b/packages/flutter/lib/src/material/text_field.dart
index 367f41aa12..3bc1393b7a 100644
--- a/packages/flutter/lib/src/material/text_field.dart
+++ b/packages/flutter/lib/src/material/text_field.dart
@@ -25,9 +25,6 @@ import 'theme.dart';
 
 export 'package:flutter/services.dart' show TextInputType, TextInputAction, TextCapitalization, SmartQuotesType, SmartDashesType;
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// Signature for the [TextField.buildCounter] callback.
 typedef InputCounterWidgetBuilder = Widget? Function(
   /// The build context for the TextField.
@@ -201,13 +198,13 @@ class _TextFieldSelectionGestureDetectorBuilder extends TextSelectionGestureDete
 /// callback. This callback is applied to the text field's current value when
 /// the user finishes editing.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 ///
 /// This sample shows how to get a value from a TextField via the [onSubmitted]
 /// callback.
 ///
 /// ```dart
-/// TextEditingController _controller;
+/// late TextEditingController _controller;
 ///
 /// void initState() {
 ///   super.initState();
@@ -769,9 +766,9 @@ class TextField extends StatefulWidget {
   /// Widget counter(
   ///   BuildContext context,
   ///   {
-  ///     int currentLength,
-  ///     int maxLength,
-  ///     bool isFocused,
+  ///     required int currentLength,
+  ///     required int? maxLength,
+  ///     required bool isFocused,
   ///   }
   /// ) {
   ///   return Text(
diff --git a/packages/flutter/lib/src/material/text_theme.dart b/packages/flutter/lib/src/material/text_theme.dart
index 3268620309..947ed76fbd 100644
--- a/packages/flutter/lib/src/material/text_theme.dart
+++ b/packages/flutter/lib/src/material/text_theme.dart
@@ -7,9 +7,6 @@ import 'package:flutter/painting.dart';
 
 import 'typography.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// Material design text theme.
 ///
 /// Definitions for the various typographical styles found in Material Design
@@ -356,7 +353,7 @@ class TextTheme with Diagnosticable {
   /// /// A Widget that sets the ambient theme's title text color for its
   /// /// descendants, while leaving other ambient theme attributes alone.
   /// class TitleColorThemeCopy extends StatelessWidget {
-  ///   TitleColorThemeCopy({Key key, this.child, this.titleColor}) : super(key: key);
+  ///   TitleColorThemeCopy({Key? key, required this.child, required this.titleColor}) : super(key: key);
   ///
   ///   final Color titleColor;
   ///   final Widget child;
@@ -367,7 +364,7 @@ class TextTheme with Diagnosticable {
   ///     return Theme(
   ///       data: theme.copyWith(
   ///         textTheme: theme.textTheme.copyWith(
-  ///           headline6: theme.textTheme.headline6.copyWith(
+  ///           headline6: theme.textTheme.headline6!.copyWith(
   ///             color: titleColor,
   ///           ),
   ///         ),
@@ -497,7 +494,7 @@ class TextTheme with Diagnosticable {
   /// /// A Widget that sets the ambient theme's title text color for its
   /// /// descendants, while leaving other ambient theme attributes alone.
   /// class TitleColorTheme extends StatelessWidget {
-  ///   TitleColorTheme({Key key, this.child, this.titleColor}) : super(key: key);
+  ///   TitleColorTheme({Key? key, required this.child, required this.titleColor}) : super(key: key);
   ///
   ///   final Color titleColor;
   ///   final Widget child;
diff --git a/packages/flutter/lib/src/material/time_picker.dart b/packages/flutter/lib/src/material/time_picker.dart
index 54e0021e3b..ac55f01555 100644
--- a/packages/flutter/lib/src/material/time_picker.dart
+++ b/packages/flutter/lib/src/material/time_picker.dart
@@ -34,8 +34,7 @@ import 'time.dart';
 import 'time_picker_theme.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// BuildContext context;
+// late BuildContext context;
 
 const Duration _kDialogSizeAnimationDuration = Duration(milliseconds: 200);
 const Duration _kDialAnimateDuration = Duration(milliseconds: 200);
@@ -2127,7 +2126,7 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
 /// Show a dialog with [initialTime] equal to the current time.
 ///
 /// ```dart
-/// Future<TimeOfDay> selectedTime = showTimePicker(
+/// Future<TimeOfDay?> selectedTime = showTimePicker(
 ///   initialTime: TimeOfDay.now(),
 ///   context: context,
 /// );
@@ -2156,13 +2155,13 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
 /// Show a dialog with the text direction overridden to be [TextDirection.rtl].
 ///
 /// ```dart
-/// Future<TimeOfDay> selectedTimeRTL = showTimePicker(
+/// Future<TimeOfDay?> selectedTimeRTL = showTimePicker(
 ///   context: context,
 ///   initialTime: TimeOfDay.now(),
-///   builder: (BuildContext context, Widget child) {
+///   builder: (BuildContext context, Widget? child) {
 ///     return Directionality(
 ///       textDirection: TextDirection.rtl,
-///       child: child,
+///       child: child!,
 ///     );
 ///   },
 /// );
@@ -2173,13 +2172,13 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
 /// Show a dialog with time unconditionally displayed in 24 hour format.
 ///
 /// ```dart
-/// Future<TimeOfDay> selectedTime24Hour = showTimePicker(
+/// Future<TimeOfDay?> selectedTime24Hour = showTimePicker(
 ///   context: context,
 ///   initialTime: TimeOfDay(hour: 10, minute: 47),
-///   builder: (BuildContext context, Widget child) {
+///   builder: (BuildContext context, Widget? child) {
 ///     return MediaQuery(
 ///       data: MediaQuery.of(context).copyWith(alwaysUse24HourFormat: true),
-///       child: child,
+///       child: child!,
 ///     );
 ///   },
 /// );