diff --git a/dev/manual_tests/lib/actions.dart b/dev/manual_tests/lib/actions.dart
index 997aa7e666..1f83b8d15e 100644
--- a/dev/manual_tests/lib/actions.dart
+++ b/dev/manual_tests/lib/actions.dart
@@ -23,9 +23,9 @@ void main() {
 /// the undo stack when they are invoked.
 class Memento extends Object with Diagnosticable {
   const Memento({
-    @required this.name,
-    @required this.undo,
-    @required this.redo,
+    required this.name,
+    required this.undo,
+    required this.redo,
   });
 
   /// Returns true if this Memento can be used to undo.
@@ -110,11 +110,11 @@ class UndoableActionDispatcher extends ActionDispatcher implements Listenable {
   }
 
   @override
-  Object invokeAction(Action<Intent> action, Intent intent, [BuildContext context]) {
-    final Object result = super.invokeAction(action, intent, context);
+  Object? invokeAction(Action<Intent> action, Intent intent, [BuildContext? context]) {
+    final Object? result = super.invokeAction(action, intent, context);
     print('Invoking ${action is UndoableAction ? 'undoable ' : ''}$intent as $action: $this ');
     if (action is UndoableAction) {
-      _completedActions.addLast(result as Memento);
+      _completedActions.addLast(result! as Memento);
       _undoneActions.clear();
       _pruneActions();
       notifyListeners();
@@ -193,14 +193,22 @@ class UndoIntent extends Intent {
 class UndoAction extends Action<UndoIntent> {
   @override
   bool isEnabled(UndoIntent intent) {
-    final UndoableActionDispatcher manager = Actions.of(primaryFocus?.context ?? FocusDemo.appKey.currentContext) as UndoableActionDispatcher;
+    final BuildContext? buildContext = primaryFocus?.context ?? FocusDemo.appKey.currentContext;
+    if (buildContext == null) {
+      return false;
+    }
+    final UndoableActionDispatcher manager = Actions.of(buildContext) as UndoableActionDispatcher;
     return manager.canUndo;
   }
 
   @override
   void invoke(UndoIntent intent) {
-    final UndoableActionDispatcher manager = Actions.of(primaryFocus?.context ?? FocusDemo.appKey.currentContext) as UndoableActionDispatcher;
-    manager?.undo();
+    final BuildContext? buildContext = primaryFocus?.context ?? FocusDemo.appKey.currentContext;
+    if (buildContext == null) {
+      return;
+    }
+    final UndoableActionDispatcher manager = Actions.of(primaryFocus?.context ?? FocusDemo.appKey.currentContext!) as UndoableActionDispatcher;
+    manager.undo();
   }
 }
 
@@ -211,14 +219,22 @@ class RedoIntent extends Intent {
 class RedoAction extends Action<RedoIntent> {
   @override
   bool isEnabled(RedoIntent intent) {
-    final UndoableActionDispatcher manager = Actions.of(primaryFocus.context) as UndoableActionDispatcher;
+    final BuildContext? buildContext = primaryFocus?.context ?? FocusDemo.appKey.currentContext;
+    if (buildContext == null) {
+      return false;
+    }
+    final UndoableActionDispatcher manager = Actions.of(buildContext) as UndoableActionDispatcher;
     return manager.canRedo;
   }
 
   @override
   RedoAction invoke(RedoIntent intent) {
-    final UndoableActionDispatcher manager = Actions.of(primaryFocus.context) as UndoableActionDispatcher;
-    manager?.redo();
+    final BuildContext? buildContext = primaryFocus?.context ?? FocusDemo.appKey.currentContext;
+    if (buildContext == null) {
+      return this;
+    }
+    final UndoableActionDispatcher manager = Actions.of(buildContext) as UndoableActionDispatcher;
+    manager.redo();
     return this;
   }
 }
@@ -226,11 +242,11 @@ class RedoAction extends Action<RedoIntent> {
 /// An action that can be undone.
 abstract class UndoableAction<T extends Intent> extends Action<T> {
   /// The [Intent] this action was originally invoked with.
-  Intent get invocationIntent => _invocationTag;
-  Intent _invocationTag;
+  Intent? get invocationIntent => _invocationTag;
+  Intent? _invocationTag;
 
   @protected
-  set invocationIntent(Intent value) => _invocationTag = value;
+  set invocationIntent(Intent? value) => _invocationTag = value;
 
   @override
   @mustCallSuper
@@ -244,8 +260,8 @@ class UndoableFocusActionBase<T extends Intent> extends UndoableAction<T> {
   @mustCallSuper
   Memento invoke(T intent) {
     super.invoke(intent);
-    final FocusNode previousFocus = primaryFocus;
-    return Memento(name: previousFocus.debugLabel, undo: () {
+    final FocusNode? previousFocus = primaryFocus;
+    return Memento(name: previousFocus!.debugLabel!, undo: () {
       previousFocus.requestFocus();
     }, redo: () {
       return invoke(intent);
@@ -267,7 +283,7 @@ class UndoableNextFocusAction extends UndoableFocusActionBase<NextFocusIntent> {
   @override
   Memento invoke(NextFocusIntent intent) {
     final Memento memento = super.invoke(intent);
-    primaryFocus.nextFocus();
+    primaryFocus?.nextFocus();
     return memento;
   }
 }
@@ -276,25 +292,25 @@ class UndoablePreviousFocusAction extends UndoableFocusActionBase<PreviousFocusI
   @override
   Memento invoke(PreviousFocusIntent intent) {
     final Memento memento = super.invoke(intent);
-    primaryFocus.previousFocus();
+    primaryFocus?.previousFocus();
     return memento;
   }
 }
 
 class UndoableDirectionalFocusAction extends UndoableFocusActionBase<DirectionalFocusIntent> {
-  TraversalDirection direction;
+  TraversalDirection? direction;
 
   @override
   Memento invoke(DirectionalFocusIntent intent) {
     final Memento memento = super.invoke(intent);
-    primaryFocus.focusInDirection(intent.direction);
+    primaryFocus?.focusInDirection(intent.direction);
     return memento;
   }
 }
 
 /// A button class that takes focus when clicked.
 class DemoButton extends StatefulWidget {
-  const DemoButton({Key key, this.name}) : super(key: key);
+  const DemoButton({Key? key, required this.name}) : super(key: key);
 
   final String name;
 
@@ -303,19 +319,13 @@ class DemoButton extends StatefulWidget {
 }
 
 class _DemoButtonState extends State<DemoButton> {
-  FocusNode _focusNode;
+  late final FocusNode _focusNode = FocusNode(debugLabel: widget.name);
   final GlobalKey _nameKey = GlobalKey();
 
-  @override
-  void initState() {
-    super.initState();
-    _focusNode = FocusNode(debugLabel: widget.name);
-  }
-
   void _handleOnPressed() {
     print('Button ${widget.name} pressed.');
     setState(() {
-      Actions.invoke(_nameKey.currentContext, RequestFocusIntent(_focusNode));
+      Actions.invoke(_nameKey.currentContext!, RequestFocusIntent(_focusNode));
     });
   }
 
@@ -336,7 +346,7 @@ class _DemoButtonState extends State<DemoButton> {
             return Colors.red;
           if (states.contains(MaterialState.hovered))
             return Colors.blue;
-          return null;
+          return Colors.transparent;
         }),
       ),
       onPressed: () => _handleOnPressed(),
@@ -346,7 +356,7 @@ class _DemoButtonState extends State<DemoButton> {
 }
 
 class FocusDemo extends StatefulWidget {
-  const FocusDemo({Key key}) : super(key: key);
+  const FocusDemo({Key? key}) : super(key: key);
 
   static GlobalKey appKey = GlobalKey();
 
@@ -355,16 +365,14 @@ class FocusDemo extends StatefulWidget {
 }
 
 class _FocusDemoState extends State<FocusDemo> {
-  FocusNode outlineFocus;
-  UndoableActionDispatcher dispatcher;
-  bool canUndo;
-  bool canRedo;
+  final FocusNode outlineFocus = FocusNode(debugLabel: 'Demo Focus Node');
+  late final UndoableActionDispatcher dispatcher = UndoableActionDispatcher();
+  bool canUndo = false;
+  bool canRedo = false;
 
   @override
   void initState() {
     super.initState();
-    outlineFocus = FocusNode(debugLabel: 'Demo Focus Node');
-    dispatcher = UndoableActionDispatcher();
     canUndo = dispatcher.canUndo;
     canRedo = dispatcher.canRedo;
     dispatcher.addListener(_handleUndoStateChange);
@@ -415,7 +423,7 @@ class _FocusDemoState extends State<FocusDemo> {
             debugLabel: 'Scope',
             autofocus: true,
             child: DefaultTextStyle(
-              style: textTheme.headline4,
+              style: textTheme.headline4!,
               child: Scaffold(
                 appBar: AppBar(
                   title: const Text('Actions Demo'),
diff --git a/dev/manual_tests/lib/animated_icons.dart b/dev/manual_tests/lib/animated_icons.dart
index ef6aa10793..2ff0e03054 100644
--- a/dev/manual_tests/lib/animated_icons.dart
+++ b/dev/manual_tests/lib/animated_icons.dart
@@ -5,7 +5,7 @@
 import 'package:flutter/material.dart';
 
 class AnimatedIconsTestApp extends StatelessWidget {
-  const AnimatedIconsTestApp({Key key}) : super(key: key);
+  const AnimatedIconsTestApp({Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
@@ -19,7 +19,7 @@ class AnimatedIconsTestApp extends StatelessWidget {
 }
 
 class IconsList extends StatelessWidget {
-  const IconsList({Key key}) : super(key: key);
+  const IconsList({Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
@@ -30,7 +30,7 @@ class IconsList extends StatelessWidget {
 }
 
 class IconSampleRow extends StatefulWidget {
-  const IconSampleRow(this.sample, {Key key}) : super(key: key);
+  const IconSampleRow(this.sample, {Key? key}) : super(key: key);
 
   final IconSample sample;
 
@@ -39,7 +39,7 @@ class IconSampleRow extends StatefulWidget {
 }
 
 class IconSampleRowState extends State<IconSampleRow> with SingleTickerProviderStateMixin {
-  AnimationController progress;
+  late final AnimationController progress = AnimationController(vsync: this, duration: const Duration(milliseconds: 300));
 
   @override
   Widget build(BuildContext context) {
@@ -63,7 +63,6 @@ class IconSampleRowState extends State<IconSampleRow> with SingleTickerProviderS
   @override
   void initState() {
     super.initState();
-    progress = AnimationController(vsync: this, duration: const Duration(milliseconds: 300));
     progress.addListener(_handleChange);
   }
 
diff --git a/dev/manual_tests/lib/card_collection.dart b/dev/manual_tests/lib/card_collection.dart
index a026fa5426..eaca4ad0b7 100644
--- a/dev/manual_tests/lib/card_collection.dart
+++ b/dev/manual_tests/lib/card_collection.dart
@@ -7,18 +7,18 @@ import 'dart:math';
 import 'package:flutter/material.dart';
 
 class CardModel {
-  CardModel(this.value, this.height) {
+  CardModel(this.value, this.height) :
     textController = TextEditingController(text: 'Item $value');
-  }
+
   int value;
   double height;
   int get color => ((value % 9) + 1) * 100;
-  TextEditingController textController;
+  final TextEditingController textController;
   Key get key => ObjectKey(this);
 }
 
 class CardCollection extends StatefulWidget {
-  const CardCollection({Key key}) : super(key: key);
+  const CardCollection({Key? key}) : super(key: key);
 
   @override
   CardCollectionState createState() => CardCollectionState();
@@ -41,7 +41,7 @@ class CardCollectionState extends State<CardCollection> {
   ];
 
   MaterialColor _primaryColor = Colors.deepPurple;
-  List<CardModel> _cardModels;
+  List<CardModel> _cardModels = <CardModel>[];
   DismissDirection _dismissDirection = DismissDirection.horizontal;
   TextAlign _textAlign = TextAlign.center;
   bool _editable = false;
@@ -164,21 +164,21 @@ class CardCollectionState extends State<CardCollection> {
     });
   }
 
-  void _selectColor(MaterialColor selection) {
+  void _selectColor(MaterialColor? selection) {
     setState(() {
-      _primaryColor = selection;
+      _primaryColor = selection!;
     });
   }
 
-  void _changeDismissDirection(DismissDirection newDismissDirection) {
+  void _changeDismissDirection(DismissDirection? newDismissDirection) {
     setState(() {
-      _dismissDirection = newDismissDirection;
+      _dismissDirection = newDismissDirection!;
     });
   }
 
-  void _changeTextAlign(TextAlign newTextAlign) {
+  void _changeTextAlign(TextAlign? newTextAlign) {
     setState(() {
-      _textAlign = newTextAlign;
+      _textAlign = newTextAlign!;
     });
   }
 
@@ -193,7 +193,7 @@ class CardCollectionState extends State<CardCollection> {
     );
   }
 
-  Widget buildDrawerColorRadioItem(String label, MaterialColor itemValue, MaterialColor currentValue, ValueChanged<MaterialColor> onChanged, { IconData icon, bool enabled = true }) {
+  Widget buildDrawerColorRadioItem(String label, MaterialColor itemValue, MaterialColor currentValue, ValueChanged<MaterialColor?> onChanged, { IconData? icon, bool enabled = true }) {
     return ListTile(
       leading: Icon(icon),
       title: Text(label),
@@ -206,7 +206,7 @@ class CardCollectionState extends State<CardCollection> {
     );
   }
 
-  Widget buildDrawerDirectionRadioItem(String label, DismissDirection itemValue, DismissDirection currentValue, ValueChanged<DismissDirection> onChanged, { IconData icon, bool enabled = true }) {
+  Widget buildDrawerDirectionRadioItem(String label, DismissDirection itemValue, DismissDirection currentValue, ValueChanged<DismissDirection?> onChanged, { IconData? icon, bool enabled = true }) {
     return ListTile(
       leading: Icon(icon),
       title: Text(label),
@@ -219,7 +219,7 @@ class CardCollectionState extends State<CardCollection> {
     );
   }
 
-  Widget buildFontRadioItem(String label, TextAlign itemValue, TextAlign currentValue, ValueChanged<TextAlign> onChanged, { IconData icon, bool enabled = true }) {
+  Widget buildFontRadioItem(String label, TextAlign itemValue, TextAlign currentValue, ValueChanged<TextAlign?> onChanged, { IconData? icon, bool enabled = true }) {
     return ListTile(
       leading: Icon(icon),
       title: Text(label),
@@ -306,7 +306,7 @@ class CardCollectionState extends State<CardCollection> {
       rightArrowIcon = Opacity(opacity: 0.1, child: rightArrowIcon);
 
     final ThemeData theme = Theme.of(context);
-    final TextStyle backgroundTextStyle = theme.primaryTextTheme.headline6;
+    final TextStyle? backgroundTextStyle = theme.primaryTextTheme.headline6;
 
     // The background Widget appears behind the Dismissible card when the card
     // moves to the left or right. The Positioned widget ensures that the
diff --git a/dev/manual_tests/lib/color_testing_demo.dart b/dev/manual_tests/lib/color_testing_demo.dart
index 9705b1c371..cbcd49fedb 100644
--- a/dev/manual_tests/lib/color_testing_demo.dart
+++ b/dev/manual_tests/lib/color_testing_demo.dart
@@ -5,7 +5,7 @@
 import 'package:flutter/material.dart';
 
 class ColorTestingDemo extends StatelessWidget {
-  const ColorTestingDemo({ Key key }) : super(key: key);
+  const ColorTestingDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/color_demo';
 
@@ -14,7 +14,7 @@ class ColorTestingDemo extends StatelessWidget {
 }
 
 class ColorDemoHome extends StatelessWidget {
-  const ColorDemoHome({Key key}) : super(key: key);
+  const ColorDemoHome({Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
@@ -52,7 +52,7 @@ class ColorDemoHome extends StatelessWidget {
 }
 
 class GradientRow extends StatelessWidget {
-  const GradientRow({ Key key, this.rightColor, this.leftColor }) : super(key: key);
+  const GradientRow({ Key? key, required this.rightColor, required this.leftColor }) : super(key: key);
 
   final Color leftColor;
   final Color rightColor;
@@ -73,7 +73,7 @@ class GradientRow extends StatelessWidget {
 }
 
 class ColorRow extends StatelessWidget {
-  const ColorRow({ Key key, this.color }) : super(key: key);
+  const ColorRow({ Key? key, required this.color }) : super(key: key);
 
   final Color color;
 
diff --git a/dev/manual_tests/lib/density.dart b/dev/manual_tests/lib/density.dart
index 1171ac9afe..16f4f7e431 100644
--- a/dev/manual_tests/lib/density.dart
+++ b/dev/manual_tests/lib/density.dart
@@ -17,12 +17,12 @@ final Map<int, Color> m2SwatchColors = <int, Color>{
   800: const Color(0xff270096),
   900: const Color(0xff270096),
 };
-final MaterialColor m2Swatch = MaterialColor(m2SwatchColors[500].value, m2SwatchColors);
+final MaterialColor m2Swatch = MaterialColor(m2SwatchColors[500]!.value, m2SwatchColors);
 
 void main() => runApp(const MyApp());
 
 class MyApp extends StatelessWidget {
-  const MyApp({Key key}) : super(key: key);
+  const MyApp({Key? key}) : super(key: key);
 
   static const String _title = 'Density Test';
 
@@ -36,7 +36,7 @@ class MyApp extends StatelessWidget {
 }
 
 class MyHomePage extends StatefulWidget {
-  const MyHomePage({Key key, this.title}) : super(key: key);
+  const MyHomePage({Key? key, required this.title}) : super(key: key);
 
   final String title;
 
@@ -112,11 +112,11 @@ class OptionModel extends ChangeNotifier {
 }
 
 class LabeledCheckbox extends StatelessWidget {
-  const LabeledCheckbox({Key key, this.label, this.onChanged, this.value}) : super(key: key);
+  const LabeledCheckbox({Key? key, required this.label, this.onChanged, this.value}) : super(key: key);
 
   final String label;
-  final ValueChanged<bool> onChanged;
-  final bool value;
+  final ValueChanged<bool?>? onChanged;
+  final bool? value;
 
   @override
   Widget build(BuildContext context) {
@@ -134,7 +134,7 @@ class LabeledCheckbox extends StatelessWidget {
 }
 
 class Options extends StatefulWidget {
-  const Options(this.model, {Key key}) : super(key: key);
+  const Options(this.model, {Key? key}) : super(key: key);
 
   final OptionModel model;
 
@@ -181,7 +181,7 @@ class _OptionsState extends State<Options> {
     return 'custom';
   }
 
-  VisualDensity _profileToDensity(String profile) {
+  VisualDensity _profileToDensity(String? profile) {
     switch (profile) {
       case 'standard':
         return VisualDensity.standard;
@@ -304,7 +304,7 @@ class _OptionsState extends State<Options> {
                     child: DropdownButton<String>(
                       style: TextStyle(color: Colors.grey[50]),
                       isDense: true,
-                      onChanged: (String value) {
+                      onChanged: (String? value) {
                         widget.model.density = _profileToDensity(value);
                       },
                       items: const <DropdownMenuItem<String>>[
@@ -321,15 +321,15 @@ class _OptionsState extends State<Options> {
                   ),
                   LabeledCheckbox(
                     label: 'Enabled',
-                    onChanged: (bool checked) {
-                      widget.model.enable = checked;
+                    onChanged: (bool? checked) {
+                      widget.model.enable = checked == true;
                     },
                     value: widget.model.enable,
                   ),
                   LabeledCheckbox(
                     label: 'Slow',
-                    onChanged: (bool checked) {
-                      widget.model.slowAnimations = checked;
+                    onChanged: (bool? checked) {
+                      widget.model.slowAnimations = checked == true;
                       Future<void>.delayed(const Duration(milliseconds: 150)).then((_) {
                         if (widget.model.slowAnimations) {
                           timeDilation = 20.0;
@@ -342,8 +342,8 @@ class _OptionsState extends State<Options> {
                   ),
                   LabeledCheckbox(
                     label: 'RTL',
-                    onChanged: (bool checked) {
-                      widget.model.rtl = checked;
+                    onChanged: (bool? checked) {
+                      widget.model.rtl = checked == true;
                     },
                     value: widget.model.rtl,
                   ),
@@ -365,7 +365,7 @@ class _OptionsState extends State<Options> {
 }
 
 class _ControlTile extends StatelessWidget {
-  const _ControlTile({Key key, @required this.label, @required this.child})
+  const _ControlTile({Key? key, required this.label, required this.child})
       : assert(label != null),
         assert(child != null),
         super(key: key);
@@ -398,13 +398,12 @@ class _ControlTile extends StatelessWidget {
 class _MyHomePageState extends State<MyHomePage> {
   static final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
   final OptionModel _model = OptionModel();
-  TextEditingController textController;
+  final TextEditingController textController = TextEditingController();
 
   @override
   void initState() {
     super.initState();
     _model.addListener(_modelChanged);
-    textController = TextEditingController();
   }
 
   @override
@@ -565,9 +564,9 @@ class _MyHomePageState extends State<MyHomePage> {
           children: List<Widget>.generate(checkboxValues.length, (int index) {
             return Checkbox(
               onChanged: _model.enable
-                  ? (bool value) {
+                  ? (bool? value) {
                       setState(() {
-                        checkboxValues[index] = value;
+                        checkboxValues[index] = value == true;
                       });
                     }
                   : null,
@@ -583,9 +582,9 @@ class _MyHomePageState extends State<MyHomePage> {
           children: List<Widget>.generate(4, (int index) {
             return Radio<int>(
               onChanged: _model.enable
-                  ? (int value) {
+                  ? (int? value) {
                       setState(() {
-                        radioValue = value;
+                        radioValue = value!;
                       });
                     }
                   : null,
diff --git a/dev/manual_tests/lib/drag_and_drop.dart b/dev/manual_tests/lib/drag_and_drop.dart
index d9d500a207..234112fdc7 100644
--- a/dev/manual_tests/lib/drag_and_drop.dart
+++ b/dev/manual_tests/lib/drag_and_drop.dart
@@ -7,7 +7,7 @@ import 'dart:math' as math;
 import 'package:flutter/material.dart';
 
 class ExampleDragTarget extends StatefulWidget {
-  const ExampleDragTarget({Key key}) : super(key: key);
+  const ExampleDragTarget({Key? key}) : super(key: key);
 
   @override
   ExampleDragTargetState createState() => ExampleDragTargetState();
@@ -26,7 +26,7 @@ class ExampleDragTargetState extends State<ExampleDragTarget> {
   Widget build(BuildContext context) {
     return DragTarget<Color>(
       onAccept: _handleAccept,
-      builder: (BuildContext context, List<Color> data, List<dynamic> rejectedData) {
+      builder: (BuildContext context, List<Color?> data, List<dynamic> rejectedData) {
         return Container(
           height: 100.0,
           margin: const EdgeInsets.all(10.0),
@@ -44,11 +44,11 @@ class ExampleDragTargetState extends State<ExampleDragTarget> {
 }
 
 class Dot extends StatefulWidget {
-  const Dot({ Key key, this.color, this.size, this.child, this.tappable = false }) : super(key: key);
+  const Dot({ Key? key, this.color, this.size, this.child, this.tappable = false }) : super(key: key);
 
-  final Color color;
-  final double size;
-  final Widget child;
+  final Color? color;
+  final double? size;
+  final Widget? child;
   final bool tappable;
 
   @override
@@ -77,17 +77,17 @@ class DotState extends State<Dot> {
 
 class ExampleDragSource extends StatelessWidget {
   const ExampleDragSource({
-    Key key,
+    Key? key,
     this.color,
     this.heavy = false,
     this.under = true,
     this.child,
   }) : super(key: key);
 
-  final Color color;
+  final Color? color;
   final bool heavy;
   final bool under;
-  final Widget child;
+  final Widget? child;
 
   static const double kDotSize = 50.0;
   static const double kHeavyMultiplier = 1.5;
@@ -100,7 +100,7 @@ class ExampleDragSource extends StatelessWidget {
       size *= kHeavyMultiplier;
 
     final Widget contents = DefaultTextStyle(
-      style: Theme.of(context).textTheme.bodyText2,
+      style: Theme.of(context).textTheme.bodyText2!,
       textAlign: TextAlign.center,
       child: Dot(
         color: color,
@@ -176,7 +176,7 @@ class DashOutlineCirclePainter extends CustomPainter {
 }
 
 class MovableBall extends StatelessWidget {
-  const MovableBall(this.position, this.ballPosition, this.callback, {Key key}) : super(key: key);
+  const MovableBall(this.position, this.ballPosition, this.callback, {Key? key}) : super(key: key);
 
   final int position;
   final int ballPosition;
@@ -188,7 +188,7 @@ class MovableBall extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final Widget ball = DefaultTextStyle(
-      style: Theme.of(context).primaryTextTheme.bodyText2,
+      style: Theme.of(context).primaryTextTheme.bodyText2!,
       textAlign: TextAlign.center,
       child: Dot(
         key: kBallKey,
@@ -216,7 +216,7 @@ class MovableBall extends StatelessWidget {
     } else {
       return DragTarget<bool>(
         onAccept: (bool data) { callback(position); },
-        builder: (BuildContext context, List<bool> accepted, List<dynamic> rejected) {
+        builder: (BuildContext context, List<bool?> accepted, List<dynamic> rejected) {
           return dashedBall;
         },
       );
@@ -225,7 +225,7 @@ class MovableBall extends StatelessWidget {
 }
 
 class DragAndDropApp extends StatefulWidget {
-  const DragAndDropApp({Key key}) : super(key: key);
+  const DragAndDropApp({Key? key}) : super(key: key);
 
   @override
   DragAndDropAppState createState() => DragAndDropAppState();
diff --git a/dev/manual_tests/lib/focus.dart b/dev/manual_tests/lib/focus.dart
index 9a6dcd7673..34b08ebc0d 100644
--- a/dev/manual_tests/lib/focus.dart
+++ b/dev/manual_tests/lib/focus.dart
@@ -13,7 +13,7 @@ void main() {
 }
 
 class DemoButton extends StatefulWidget {
-  const DemoButton({Key key, this.name, this.canRequestFocus = true, this.autofocus = false}) : super(key: key);
+  const DemoButton({Key? key, required this.name, this.canRequestFocus = true, this.autofocus = false}) : super(key: key);
 
   final String name;
   final bool canRequestFocus;
@@ -24,20 +24,14 @@ class DemoButton extends StatefulWidget {
 }
 
 class _DemoButtonState extends State<DemoButton> {
-  FocusNode focusNode;
-
-  @override
-  void initState() {
-    super.initState();
-    focusNode = FocusNode(
+  late final FocusNode focusNode = FocusNode(
       debugLabel: widget.name,
       canRequestFocus: widget.canRequestFocus,
-    );
-  }
+  );
 
   @override
   void dispose() {
-    focusNode?.dispose();
+    focusNode.dispose();
     super.dispose();
   }
 
@@ -64,7 +58,7 @@ class _DemoButtonState extends State<DemoButton> {
             return Colors.red.withOpacity(0.25);
           if (states.contains(MaterialState.hovered))
             return Colors.blue.withOpacity(0.25);
-          return null;
+          return Colors.transparent;
         }),
       ),
       onPressed: () => _handleOnPressed(),
@@ -74,14 +68,14 @@ class _DemoButtonState extends State<DemoButton> {
 }
 
 class FocusDemo extends StatefulWidget {
-  const FocusDemo({Key key}) : super(key: key);
+  const FocusDemo({Key? key}) : super(key: key);
 
   @override
   State<FocusDemo> createState() => _FocusDemoState();
 }
 
 class _FocusDemoState extends State<FocusDemo> {
-  FocusNode outlineFocus;
+  FocusNode? outlineFocus;
 
   @override
   void initState() {
@@ -91,7 +85,7 @@ class _FocusDemoState extends State<FocusDemo> {
 
   @override
   void dispose() {
-    outlineFocus.dispose();
+    outlineFocus?.dispose();
     super.dispose();
   }
 
@@ -142,7 +136,7 @@ class _FocusDemoState extends State<FocusDemo> {
         onKey: _handleKeyPress,
         autofocus: true,
         child: DefaultTextStyle(
-          style: textTheme.headline4,
+          style: textTheme.headline4!,
           child: Scaffold(
             appBar: AppBar(
               title: const Text('Focus Demo'),
diff --git a/dev/manual_tests/lib/hover.dart b/dev/manual_tests/lib/hover.dart
index b040b8c0ea..788dfdd31d 100644
--- a/dev/manual_tests/lib/hover.dart
+++ b/dev/manual_tests/lib/hover.dart
@@ -12,7 +12,7 @@ void main() {
 }
 
 class DemoButton extends StatelessWidget {
-  const DemoButton({Key key, this.name}) : super(key: key);
+  const DemoButton({Key? key, required this.name}) : super(key: key);
 
   final String name;
 
@@ -30,7 +30,7 @@ class DemoButton extends StatelessWidget {
 }
 
 class HoverDemo extends StatefulWidget {
-  const HoverDemo({Key key}) : super(key: key);
+  const HoverDemo({Key? key}) : super(key: key);
 
   @override
   State<HoverDemo> createState() => _HoverDemoState();
@@ -42,12 +42,12 @@ class _HoverDemoState extends State<HoverDemo> {
     final TextTheme textTheme = Theme.of(context).textTheme;
     final ButtonStyle overrideFocusColor = ButtonStyle(
       overlayColor: MaterialStateProperty.resolveWith<Color>((Set<MaterialState> states) {
-        return states.contains(MaterialState.focused) ? Colors.deepOrangeAccent : null;
+        return states.contains(MaterialState.focused) ? Colors.deepOrangeAccent : Colors.transparent;
       })
     );
 
     return DefaultTextStyle(
-      style: textTheme.headline4,
+      style: textTheme.headline4!,
       child: Scaffold(
         appBar: AppBar(
           title: const Text('Hover Demo'),
diff --git a/dev/manual_tests/lib/material_arc.dart b/dev/manual_tests/lib/material_arc.dart
index 040ef737c1..571a5b6151 100644
--- a/dev/manual_tests/lib/material_arc.dart
+++ b/dev/manual_tests/lib/material_arc.dart
@@ -45,12 +45,12 @@ class _IgnoreDrag extends Drag {
 
 class _PointDemoPainter extends CustomPainter {
   _PointDemoPainter({
-    Animation<double> repaint,
-    this.arc,
+    Animation<double>? repaint,
+    required this.arc,
   }) : _repaint = repaint, super(repaint: repaint);
 
   final MaterialPointArcTween arc;
-  final Animation<double> _repaint;
+  final Animation<double>? _repaint;
 
   void drawPoint(Canvas canvas, Offset point, Color color) {
     final Paint paint = Paint()
@@ -69,7 +69,7 @@ class _PointDemoPainter extends CustomPainter {
     final Paint paint = Paint();
 
     if (arc.center != null)
-      drawPoint(canvas, arc.center, Colors.grey.shade400);
+      drawPoint(canvas, arc.center!, Colors.grey.shade400);
 
     paint
       ..isAntiAlias = false // Work-around for github.com/flutter/flutter/issues/5720
@@ -77,23 +77,23 @@ class _PointDemoPainter extends CustomPainter {
       ..strokeWidth = 4.0
       ..style = PaintingStyle.stroke;
     if (arc.center != null && arc.radius != null)
-      canvas.drawCircle(arc.center, arc.radius, paint);
+      canvas.drawCircle(arc.center!, arc.radius!, paint);
     else
-      canvas.drawLine(arc.begin, arc.end, paint);
+      canvas.drawLine(arc.begin!, arc.end!, paint);
 
-    drawPoint(canvas, arc.begin, Colors.green);
-    drawPoint(canvas, arc.end, Colors.red);
+    drawPoint(canvas, arc.begin!, Colors.green);
+    drawPoint(canvas, arc.end!, Colors.red);
 
     paint
       ..color = Colors.green
       ..style = PaintingStyle.fill;
-    canvas.drawCircle(arc.lerp(_repaint.value), _kPointRadius, paint);
+    canvas.drawCircle(arc.lerp(_repaint!.value), _kPointRadius, paint);
   }
 
   @override
   bool hitTest(Offset position) {
-    return (arc.begin - position).distanceSquared < _kTargetSlop
-        || (arc.end - position).distanceSquared < _kTargetSlop;
+    return (arc.begin! - position).distanceSquared < _kTargetSlop
+        || (arc.end! - position).distanceSquared < _kTargetSlop;
   }
 
   @override
@@ -101,7 +101,7 @@ class _PointDemoPainter extends CustomPainter {
 }
 
 class _PointDemo extends StatefulWidget {
-  const _PointDemo({ Key key, this.controller }) : super(key: key);
+  const _PointDemo({ Key? key, required this.controller }) : super(key: key);
 
   final AnimationController controller;
 
@@ -112,11 +112,11 @@ class _PointDemo extends StatefulWidget {
 class _PointDemoState extends State<_PointDemo> {
   final GlobalKey _painterKey = GlobalKey();
 
-  CurvedAnimation _animation;
-  _DragTarget _dragTarget;
-  Size _screenSize;
-  Offset _begin;
-  Offset _end;
+  CurvedAnimation? _animation;
+  _DragTarget? _dragTarget;
+  Size? _screenSize;
+  Offset? _begin;
+  Offset? _end;
 
   @override
   void initState() {
@@ -135,9 +135,9 @@ class _PointDemoState extends State<_PointDemo> {
     if (_dragTarget != null)
       return _IgnoreDrag();
 
-    final RenderBox box = _painterKey.currentContext.findRenderObject() as RenderBox;
-    final double startOffset = (box.localToGlobal(_begin) - position).distanceSquared;
-    final double endOffset = (box.localToGlobal(_end) - position).distanceSquared;
+    final RenderBox? box = _painterKey.currentContext!.findRenderObject() as RenderBox?;
+    final double startOffset = (box!.localToGlobal(_begin!) - position).distanceSquared;
+    final double endOffset = (box.localToGlobal(_end!) - position).distanceSquared;
     setState(() {
       if (startOffset < endOffset && startOffset < _kTargetSlop)
         _dragTarget = _DragTarget.start;
@@ -151,15 +151,15 @@ class _PointDemoState extends State<_PointDemo> {
   }
 
   void _handleDragUpdate(DragUpdateDetails details) {
-    switch (_dragTarget) {
+    switch (_dragTarget!) {
       case _DragTarget.start:
         setState(() {
-          _begin = _begin + details.delta;
+          _begin = _begin! + details.delta;
         });
         break;
       case _DragTarget.end:
         setState(() {
-          _end = _end + details.delta;
+          _end = _end! + details.delta;
         });
         break;
     }
@@ -210,7 +210,7 @@ class _PointDemoState extends State<_PointDemo> {
               child: Text(
                 'Tap the refresh button to run the animation. Drag the green '
                 "and red points to change the animation's path.",
-                style: Theme.of(context).textTheme.caption.copyWith(fontSize: 16.0),
+                style: Theme.of(context).textTheme.caption?.copyWith(fontSize: 16.0),
               ),
             ),
           ),
@@ -222,8 +222,8 @@ class _PointDemoState extends State<_PointDemo> {
 
 class _RectangleDemoPainter extends CustomPainter {
   _RectangleDemoPainter({
-    Animation<double> repaint,
-    this.arc,
+    required Animation<double> repaint,
+    required this.arc,
   }) : _repaint = repaint, super(repaint: repaint);
 
   final MaterialRectArcTween arc;
@@ -252,15 +252,15 @@ class _RectangleDemoPainter extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
-    drawRect(canvas, arc.begin, Colors.green);
-    drawRect(canvas, arc.end, Colors.red);
+    drawRect(canvas, arc.begin!, Colors.green);
+    drawRect(canvas, arc.end!, Colors.red);
     drawRect(canvas, arc.lerp(_repaint.value), Colors.blue);
   }
 
   @override
   bool hitTest(Offset position) {
-    return (arc.begin.center - position).distanceSquared < _kTargetSlop
-        || (arc.end.center - position).distanceSquared < _kTargetSlop;
+    return (arc.begin!.center - position).distanceSquared < _kTargetSlop
+        || (arc.end!.center - position).distanceSquared < _kTargetSlop;
   }
 
   @override
@@ -268,7 +268,7 @@ class _RectangleDemoPainter extends CustomPainter {
 }
 
 class _RectangleDemo extends StatefulWidget {
-  const _RectangleDemo({ Key key, this.controller }) : super(key: key);
+  const _RectangleDemo({ Key? key, required this.controller }) : super(key: key);
 
   final AnimationController controller;
 
@@ -279,17 +279,11 @@ class _RectangleDemo extends StatefulWidget {
 class _RectangleDemoState extends State<_RectangleDemo> {
   final GlobalKey _painterKey = GlobalKey();
 
-  CurvedAnimation _animation;
-  _DragTarget _dragTarget;
-  Size _screenSize;
-  Rect _begin;
-  Rect _end;
-
-  @override
-  void initState() {
-    super.initState();
-    _animation = CurvedAnimation(parent: widget.controller, curve: Curves.fastOutSlowIn);
-  }
+  late final CurvedAnimation _animation = CurvedAnimation(parent: widget.controller, curve: Curves.fastOutSlowIn);
+  _DragTarget? _dragTarget;
+  Size? _screenSize;
+  Rect? _begin;
+  Rect? _end;
 
   @override
   void dispose() {
@@ -302,9 +296,9 @@ class _RectangleDemoState extends State<_RectangleDemo> {
     if (_dragTarget != null)
       return _IgnoreDrag();
 
-    final RenderBox box = _painterKey.currentContext.findRenderObject() as RenderBox;
-    final double startOffset = (box.localToGlobal(_begin.center) - position).distanceSquared;
-    final double endOffset = (box.localToGlobal(_end.center) - position).distanceSquared;
+    final RenderBox? box = _painterKey.currentContext?.findRenderObject() as RenderBox?;
+    final double startOffset = (box!.localToGlobal(_begin!.center) - position).distanceSquared;
+    final double endOffset = (box.localToGlobal(_end!.center) - position).distanceSquared;
     setState(() {
       if (startOffset < endOffset && startOffset < _kTargetSlop)
         _dragTarget = _DragTarget.start;
@@ -317,15 +311,15 @@ class _RectangleDemoState extends State<_RectangleDemo> {
   }
 
   void _handleDragUpdate(DragUpdateDetails details) {
-    switch (_dragTarget) {
+    switch (_dragTarget!) {
       case _DragTarget.start:
         setState(() {
-          _begin = _begin.shift(details.delta);
+          _begin = _begin?.shift(details.delta);
         });
         break;
       case _DragTarget.end:
         setState(() {
-          _end = _end.shift(details.delta);
+          _end = _end?.shift(details.delta);
         });
         break;
     }
@@ -382,7 +376,7 @@ class _RectangleDemoState extends State<_RectangleDemo> {
               child: Text(
                 'Tap the refresh button to run the animation. Drag the rectangles '
                 "to change the animation's path.",
-                style: Theme.of(context).textTheme.caption.copyWith(fontSize: 16.0),
+                style: Theme.of(context).textTheme.caption!.copyWith(fontSize: 16.0),
               ),
             ),
           ),
@@ -406,37 +400,31 @@ class _ArcDemo {
 }
 
 class AnimationDemo extends StatefulWidget {
-  const AnimationDemo({ Key key }) : super(key: key);
+  const AnimationDemo({ Key? key }) : super(key: key);
 
   @override
   State<AnimationDemo> createState() => _AnimationDemoState();
 }
 
 class _AnimationDemoState extends State<AnimationDemo> with TickerProviderStateMixin {
-  List<_ArcDemo> _allDemos;
-
-  @override
-  void initState() {
-    super.initState();
-    _allDemos = <_ArcDemo>[
-      _ArcDemo('POINT', (_ArcDemo demo) {
-        return _PointDemo(
-          key: demo.key,
-          controller: demo.controller,
-        );
-      }, this),
-      _ArcDemo('RECTANGLE', (_ArcDemo demo) {
-        return _RectangleDemo(
-          key: demo.key,
-          controller: demo.controller,
-        );
-      }, this),
-    ];
-  }
+  late final List<_ArcDemo> _allDemos = <_ArcDemo>[
+    _ArcDemo('POINT', (_ArcDemo demo) {
+      return _PointDemo(
+        key: demo.key,
+        controller: demo.controller,
+      );
+    }, this),
+    _ArcDemo('RECTANGLE', (_ArcDemo demo) {
+      return _RectangleDemo(
+        key: demo.key,
+        controller: demo.controller,
+      );
+    }, this),
+  ];
 
   Future<void> _play(_ArcDemo demo) async {
     await demo.controller.forward();
-    if (demo.key.currentState != null && demo.key.currentState.mounted)
+    if (demo.key.currentState != null && demo.key.currentState!.mounted)
       demo.controller.reverse();
   }
 
@@ -456,7 +444,7 @@ class _AnimationDemoState extends State<AnimationDemo> with TickerProviderStateM
             return FloatingActionButton(
               child: const Icon(Icons.refresh),
               onPressed: () {
-                _play(_allDemos[DefaultTabController.of(context).index]);
+                _play(_allDemos[DefaultTabController.of(context)!.index]);
               },
             );
           },
diff --git a/dev/manual_tests/lib/overlay_geometry.dart b/dev/manual_tests/lib/overlay_geometry.dart
index 5ecf4e5673..c224eb273b 100644
--- a/dev/manual_tests/lib/overlay_geometry.dart
+++ b/dev/manual_tests/lib/overlay_geometry.dart
@@ -20,8 +20,8 @@ enum MarkerType { topLeft, bottomRight, touch }
 
 class _MarkerPainter extends CustomPainter {
   const _MarkerPainter({
-    this.size,
-    this.type,
+    required this.size,
+    required this.type,
   });
 
   final double size;
@@ -56,21 +56,21 @@ class _MarkerPainter extends CustomPainter {
 
 class Marker extends StatelessWidget {
   const Marker({
-    Key key,
+    Key? key,
     this.type = MarkerType.touch,
     this.position,
     this.size = 40.0,
   }) : super(key: key);
 
-  final Offset position;
+  final Offset? position;
   final double size;
   final MarkerType type;
 
   @override
   Widget build(BuildContext context) {
     return Positioned(
-      left: position.dx - size / 2.0,
-      top: position.dy - size / 2.0,
+      left: position!.dx - size / 2.0,
+      top: position!.dy - size / 2.0,
       width: size,
       height: size,
       child: IgnorePointer(
@@ -86,7 +86,7 @@ class Marker extends StatelessWidget {
 }
 
 class OverlayGeometryApp extends StatefulWidget {
-  const OverlayGeometryApp({Key key}) : super(key: key);
+  const OverlayGeometryApp({Key? key}) : super(key: key);
 
   @override
   OverlayGeometryAppState createState() => OverlayGeometryAppState();
@@ -95,22 +95,22 @@ class OverlayGeometryApp extends StatefulWidget {
 typedef CardTapCallback = void Function(GlobalKey targetKey, Offset globalPosition);
 
 class CardBuilder extends SliverChildDelegate {
-  CardBuilder({ this.cardModels, this.onTapUp });
+  CardBuilder({List<CardModel>? cardModels, this.onTapUp }) : cardModels = cardModels ?? <CardModel>[];
 
   final List<CardModel> cardModels;
-  final CardTapCallback onTapUp;
+  final CardTapCallback? onTapUp;
 
   static const TextStyle cardLabelStyle =
     TextStyle(color: Colors.white, fontSize: 18.0, fontWeight: FontWeight.bold);
 
   @override
-  Widget build(BuildContext context, int index) {
+  Widget? build(BuildContext context, int index) {
     if (index >= cardModels.length)
       return null;
     final CardModel cardModel = cardModels[index];
     return GestureDetector(
       key: cardModel.key,
-      onTapUp: (TapUpDetails details) { onTapUp(cardModel.targetKey, details.globalPosition); },
+      onTapUp: (TapUpDetails details) { onTapUp!(cardModel.targetKey, details.globalPosition); },
       child: Card(
         key: cardModel.targetKey,
         color: cardModel.color,
@@ -133,7 +133,7 @@ class CardBuilder extends SliverChildDelegate {
 }
 
 class OverlayGeometryAppState extends State<OverlayGeometryApp> {
-  List<CardModel> cardModels;
+  List<CardModel> cardModels = <CardModel>[];
   Map<MarkerType, Offset> markers = <MarkerType, Offset>{};
   double markersScrollOffset = 0.0;
 
@@ -146,8 +146,8 @@ class OverlayGeometryAppState extends State<OverlayGeometryApp> {
       48.0, 63.0, 82.0, 146.0, 60.0, 55.0, 84.0, 96.0, 50.0,
     ];
     cardModels = List<CardModel>.generate(cardHeights.length, (int i) {
-      final Color color = Color.lerp(Colors.red.shade300, Colors.blue.shade900, i / cardHeights.length);
-      return CardModel(i, cardHeights[i], color);
+      final Color? color = Color.lerp(Colors.red.shade300, Colors.blue.shade900, i / cardHeights.length);
+      return CardModel(i, cardHeights[i], color!);
     });
   }
 
@@ -156,10 +156,9 @@ class OverlayGeometryAppState extends State<OverlayGeometryApp> {
       setState(() {
         final double dy = markersScrollOffset - notification.metrics.extentBefore;
         markersScrollOffset = notification.metrics.extentBefore;
-        for (final MarkerType type in markers.keys) {
-          final Offset oldPosition = markers[type];
+        markers.forEach((MarkerType type, Offset oldPosition) {
           markers[type] = oldPosition.translate(0.0, dy);
-        }
+        });
       });
     }
     return false;
@@ -168,12 +167,12 @@ class OverlayGeometryAppState extends State<OverlayGeometryApp> {
   void handleTapUp(GlobalKey target, Offset globalPosition) {
     setState(() {
       markers[MarkerType.touch] = globalPosition;
-      final RenderBox box = target.currentContext.findRenderObject() as RenderBox;
-      markers[MarkerType.topLeft] = box.localToGlobal(Offset.zero);
+      final RenderBox? box = target.currentContext?.findRenderObject() as RenderBox?;
+      markers[MarkerType.topLeft] = box!.localToGlobal(Offset.zero);
       final Size size = box.size;
       markers[MarkerType.bottomRight] = box.localToGlobal(Offset(size.width, size.height));
-      final ScrollableState scrollable = Scrollable.of(target.currentContext);
-      markersScrollOffset = scrollable.position.pixels;
+      final ScrollableState? scrollable = Scrollable.of(target.currentContext!);
+      markersScrollOffset = scrollable!.position.pixels;
     });
   }
 
diff --git a/dev/manual_tests/lib/page_view.dart b/dev/manual_tests/lib/page_view.dart
index f6c1a91767..86ff201fcd 100644
--- a/dev/manual_tests/lib/page_view.dart
+++ b/dev/manual_tests/lib/page_view.dart
@@ -14,7 +14,7 @@ class CardModel {
 }
 
 class PageViewApp extends StatefulWidget {
-  const PageViewApp({Key key}) : super(key: key);
+  const PageViewApp({Key? key}) : super(key: key);
 
   @override
   PageViewAppState createState() => PageViewAppState();
@@ -33,15 +33,15 @@ class PageViewAppState extends State<PageViewApp> {
     ];
 
     cardModels = List<CardModel>.generate(cardSizes.length, (int i) {
-      final Color color = Color.lerp(Colors.red.shade300, Colors.blue.shade900, i / cardSizes.length);
-      return CardModel(i, cardSizes[i], color);
+      final Color? color = Color.lerp(Colors.red.shade300, Colors.blue.shade900, i / cardSizes.length);
+      return CardModel(i, cardSizes[i], color!);
     });
   }
 
   static const TextStyle cardLabelStyle =
     TextStyle(color: Colors.white, fontSize: 18.0, fontWeight: FontWeight.bold);
 
-  List<CardModel> cardModels;
+  List<CardModel> cardModels = <CardModel>[];
   Size pageSize = const Size(200.0, 200.0);
   Axis scrollDirection = Axis.horizontal;
   bool itemsWrap = false;
diff --git a/dev/manual_tests/lib/raw_keyboard.dart b/dev/manual_tests/lib/raw_keyboard.dart
index c34e00cdb4..eba88f8025 100644
--- a/dev/manual_tests/lib/raw_keyboard.dart
+++ b/dev/manual_tests/lib/raw_keyboard.dart
@@ -20,7 +20,7 @@ void main() {
 }
 
 class RawKeyboardDemo extends StatefulWidget {
-  const RawKeyboardDemo({Key key}) : super(key: key);
+  const RawKeyboardDemo({Key? key}) : super(key: key);
 
   @override
   State<RawKeyboardDemo> createState() => _HardwareKeyDemoState();
@@ -28,7 +28,7 @@ class RawKeyboardDemo extends StatefulWidget {
 
 class _HardwareKeyDemoState extends State<RawKeyboardDemo> {
   final FocusNode _focusNode = FocusNode();
-  RawKeyEvent _event;
+  RawKeyEvent? _event;
 
   @override
   void dispose() {
@@ -60,7 +60,7 @@ class _HardwareKeyDemoState extends State<RawKeyboardDemo> {
       autofocus: true,
       child: AnimatedBuilder(
         animation: _focusNode,
-        builder: (BuildContext context, Widget child) {
+        builder: (BuildContext context, Widget? child) {
           if (!_focusNode.hasFocus) {
             return GestureDetector(
               onTap: () {
@@ -74,11 +74,11 @@ class _HardwareKeyDemoState extends State<RawKeyboardDemo> {
             return Text('Press a key', style: textTheme.headline4);
           }
 
-          final RawKeyEventData data = _event.data;
-          final String modifierList = data.modifiersPressed.keys.map<String>(_getEnumName).join(', ').replaceAll('Modifier', '');
+          final RawKeyEventData? data = _event?.data;
+          final String? modifierList = data?.modifiersPressed.keys.map<String>(_getEnumName).join(', ').replaceAll('Modifier', '');
           final List<Widget> dataText = <Widget>[
             Text('${_event.runtimeType}'),
-            if (_event.character?.isNotEmpty ?? false) Text('character produced: "${_event.character}"'),
+            if (_event?.character?.isNotEmpty ?? false) Text('character produced: "${_event?.character}"'),
             Text('modifiers set: $modifierList'),
           ];
           if (data is RawKeyEventDataAndroid) {
@@ -119,12 +119,12 @@ class _HardwareKeyDemoState extends State<RawKeyboardDemo> {
             dataText.add(Text('code: ${data.code}'));
             dataText.add(Text('metaState: ${data.metaState} (${_asHex(data.metaState)})'));
           }
-          dataText.add(Text('logical: ${_event.logicalKey}'));
-          dataText.add(Text('physical: ${_event.physicalKey}'));
-          if (_event.character != null) {
-            dataText.add(Text('character: ${_event.character}'));
+          dataText.add(Text('logical: ${_event?.logicalKey}'));
+          dataText.add(Text('physical: ${_event?.physicalKey}'));
+          if (_event?.character != null) {
+            dataText.add(Text('character: ${_event?.character}'));
           }
-          for (final ModifierKey modifier in data.modifiersPressed.keys) {
+          for (final ModifierKey modifier in data!.modifiersPressed.keys) {
             for (final KeyboardSide side in KeyboardSide.values) {
               if (data.isModifierPressed(modifier, side: side)) {
                 dataText.add(
@@ -135,11 +135,11 @@ class _HardwareKeyDemoState extends State<RawKeyboardDemo> {
           }
           final List<String> pressed = <String>['Pressed:'];
           for (final LogicalKeyboardKey key in RawKeyboard.instance.keysPressed) {
-            pressed.add(key.debugName);
+            pressed.add(key.debugName!);
           }
           dataText.add(Text(pressed.join(' ')));
           return DefaultTextStyle(
-            style: textTheme.subtitle1,
+            style: textTheme.subtitle1!,
             child: Column(
               mainAxisAlignment: MainAxisAlignment.center,
               children: dataText,
diff --git a/dev/manual_tests/lib/text.dart b/dev/manual_tests/lib/text.dart
index f88f851e0c..82a6d33eac 100644
--- a/dev/manual_tests/lib/text.dart
+++ b/dev/manual_tests/lib/text.dart
@@ -26,7 +26,7 @@ void main() {
 }
 
 class Home extends StatefulWidget {
-  const Home({ Key key }) : super(key: key);
+  const Home({ Key? key }) : super(key: key);
 
   @override
   State<Home> createState() => _HomeState();
@@ -119,7 +119,7 @@ class _HomeState extends State<Home> {
 }
 
 class Fuzzer extends StatefulWidget {
-  const Fuzzer({ Key key, this.seed }) : super(key: key);
+  const Fuzzer({ Key? key, required this.seed }) : super(key: key);
 
   final int seed;
 
@@ -129,14 +129,12 @@ class Fuzzer extends StatefulWidget {
 
 class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
   TextSpan _textSpan = const TextSpan(text: 'Welcome to the Flutter text fuzzer.');
-  Ticker _ticker;
-  math.Random _random;
+  late final Ticker _ticker = createTicker(_updateTextSpan)..start();
+  late final math.Random _random = math.Random(widget.seed); // providing a seed is important for reproducibility;
 
   @override
   void initState() {
     super.initState();
-    _random = math.Random(widget.seed); // providing a seed is important for reproducibility
-    _ticker = createTicker(_updateTextSpan)..start();
     _updateTextSpan(null);
   }
 
@@ -146,7 +144,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     super.dispose();
   }
 
-  void _updateTextSpan(Duration duration) {
+  void _updateTextSpan(Duration? duration) {
     setState(() {
       _textSpan = _fiddleWith(_textSpan);
     });
@@ -156,17 +154,17 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     return TextSpan(
       text: _fiddleWithText(node.text),
       style: _fiddleWithStyle(node.style),
-      children: _fiddleWithChildren(node.children?.map((InlineSpan child) => _fiddleWith(child as TextSpan))?.toList() ?? <TextSpan>[]),
+      children: _fiddleWithChildren(node.children?.map((InlineSpan child) => _fiddleWith(child as TextSpan)).toList() ?? <TextSpan>[]),
     );
   }
 
-  String _fiddleWithText(String text) {
+  String? _fiddleWithText(String? text) {
     if (_random.nextInt(10) > 0)
       return text;
     return _createRandomText();
   }
 
-  TextStyle _fiddleWithStyle(TextStyle style) {
+  TextStyle? _fiddleWithStyle(TextStyle? style) {
     if (style == null) {
       switch (_random.nextInt(20)) {
         case 0:
@@ -196,7 +194,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     );
   }
 
-  Color _fiddleWithColor(Color value) {
+  Color? _fiddleWithColor(Color? value) {
     switch (_random.nextInt(10)) {
       case 0:
         if (value == null)
@@ -218,7 +216,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     return value;
   }
 
-  TextDecoration _fiddleWithDecoration(TextDecoration value) {
+  TextDecoration? _fiddleWithDecoration(TextDecoration? value) {
     if (_random.nextInt(10) > 0)
       return value;
     switch (_random.nextInt(100)) {
@@ -246,7 +244,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     return null;
   }
 
-  TextDecorationStyle _fiddleWithDecorationStyle(TextDecorationStyle value) {
+  TextDecorationStyle? _fiddleWithDecorationStyle(TextDecorationStyle? value) {
     switch (_random.nextInt(10)) {
       case 0:
         return null;
@@ -256,7 +254,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     return value;
   }
 
-  FontWeight _fiddleWithFontWeight(FontWeight value) {
+  FontWeight? _fiddleWithFontWeight(FontWeight? value) {
     switch (_random.nextInt(10)) {
       case 0:
         return null;
@@ -266,7 +264,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     return value;
   }
 
-  FontStyle _fiddleWithFontStyle(FontStyle value) {
+  FontStyle? _fiddleWithFontStyle(FontStyle? value) {
     switch (_random.nextInt(10)) {
       case 0:
         return null;
@@ -276,7 +274,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     return value;
   }
 
-  String _fiddleWithFontFamily(String value) {
+  String? _fiddleWithFontFamily(String? value) {
     switch (_random.nextInt(10)) {
       case 0:
         return null;
@@ -300,7 +298,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     return value;
   }
 
-  double _fiddleWithDouble(double value, double defaultValue, double max) {
+  double? _fiddleWithDouble(double? value, double defaultValue, double max) {
     switch (_random.nextInt(10)) {
       case 0:
         if (value == null)
@@ -312,7 +310,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     return value;
   }
 
-  List<TextSpan> _fiddleWithChildren(List<TextSpan> children) {
+  List<TextSpan>? _fiddleWithChildren(List<TextSpan> children) {
     switch (_random.nextInt(100)) {
       case 0:
       case 1:
@@ -340,10 +338,10 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
   }
 
   int depthOf(TextSpan node) {
-    if (node.children == null || node.children.isEmpty)
+    if (node.children == null || node.children?.isEmpty == true)
       return 0;
     int result = 0;
-    for (final TextSpan child in node.children.cast<TextSpan>())
+    for (final TextSpan child in node.children!.cast<TextSpan>())
       result = math.max(result, depthOf(child));
     return result;
   }
@@ -354,7 +352,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
     );
   }
 
-  String _createRandomText() {
+  String? _createRandomText() {
     switch (_random.nextInt(90)) {
       case 0:
       case 1:
@@ -536,7 +534,7 @@ class _FuzzerState extends State<Fuzzer> with SingleTickerProviderStateMixin {
 }
 
 class Underlines extends StatefulWidget {
-  const Underlines({ Key key }) : super(key: key);
+  const Underlines({ Key? key }) : super(key: key);
 
   @override
   State<Underlines> createState() => _UnderlinesState();
@@ -554,7 +552,7 @@ class _UnderlinesState extends State<Underlines> {
     decorationColor: Colors.yellow.shade500,
   );
 
-  Widget _wrap(TextDecorationStyle style) {
+  Widget _wrap(TextDecorationStyle? style) {
     return Align(
       alignment: Alignment.centerLeft,
       heightFactor: 1.0,
@@ -641,7 +639,7 @@ class _UnderlinesState extends State<Underlines> {
 }
 
 class Fallback extends StatefulWidget {
-  const Fallback({ Key key }) : super(key: key);
+  const Fallback({ Key? key }) : super(key: key);
 
   @override
   State<Fallback> createState() => _FallbackState();
@@ -736,7 +734,7 @@ class _FallbackState extends State<Fallback> {
 }
 
 class Bidi extends StatefulWidget {
-  const Bidi({ Key key }) : super(key: key);
+  const Bidi({ Key? key }) : super(key: key);
 
   @override
   State<Bidi> createState() => _BidiState();
@@ -811,7 +809,7 @@ class _BidiState extends State<Bidi> {
 }
 
 class Zalgo extends StatefulWidget {
-  const Zalgo({ Key key, this.seed }) : super(key: key);
+  const Zalgo({ Key? key, required this.seed }) : super(key: key);
 
   final int seed;
 
@@ -820,15 +818,14 @@ class Zalgo extends StatefulWidget {
 }
 
 class _ZalgoState extends State<Zalgo> with SingleTickerProviderStateMixin {
-  String _text;
-  Ticker _ticker;
-  math.Random _random;
+  String? _text;
+  late final Ticker _ticker = createTicker(_update)..start();
+  math.Random _random = math.Random();
 
   @override
   void initState() {
     super.initState();
-    _random = math.Random(widget.seed); // providing a seed is important for reproducibility
-    _ticker = createTicker(_update)..start();
+    _random = math.Random(widget.seed); // providing a seed is important for reproducibility;
     _update(null);
   }
 
@@ -841,7 +838,7 @@ class _ZalgoState extends State<Zalgo> with SingleTickerProviderStateMixin {
   bool _allowSpacing = false;
   bool _varyBase = false;
 
-  void _update(Duration duration) {
+  void _update(Duration? duration) {
     setState(() {
       _text = zalgo(
         _random,
@@ -918,7 +915,7 @@ class _ZalgoState extends State<Zalgo> with SingleTickerProviderStateMixin {
 }
 
 class Painting extends StatefulWidget {
-  const Painting({ Key key, this.seed }) : super(key: key);
+  const Painting({ Key? key, required this.seed }) : super(key: key);
 
   final int seed;
 
@@ -927,15 +924,14 @@ class Painting extends StatefulWidget {
 }
 
 class _PaintingState extends State<Painting> with SingleTickerProviderStateMixin {
-  String _text;
-  Ticker _ticker;
-  math.Random _random;
+  String? _text;
+  late final Ticker _ticker = createTicker(_update)..start();
+  math.Random _random = math.Random();
 
   @override
   void initState() {
     super.initState();
-    _random = math.Random(widget.seed); // providing a seed is important for reproducibility
-    _ticker = createTicker(_update)..start();
+    _random = math.Random(widget.seed); // providing a seed is important for reproducibility;
     _update(null);
   }
 
@@ -950,7 +946,7 @@ class _PaintingState extends State<Painting> with SingleTickerProviderStateMixin
 
   bool _ellipsize = false;
 
-  void _update(Duration duration) {
+  void _update(Duration? duration) {
     setState(() {
       final StringBuffer buffer = StringBuffer();
       final int targetLength = _random.nextInt(20) + (_ellipsize ? MediaQuery.of(context).size.width.round() : 1);
@@ -963,11 +959,11 @@ class _PaintingState extends State<Painting> with SingleTickerProviderStateMixin
       }
       _text = buffer.toString();
     });
-    SchedulerBinding.instance.addPostFrameCallback((Duration duration) {
-      if (mounted && intrinsicKey.currentContext.size.height != controlKey.currentContext.size.height) {
+    SchedulerBinding.instance?.addPostFrameCallback((Duration duration) {
+      if (mounted && intrinsicKey.currentContext?.size?.height != controlKey.currentContext?.size?.height) {
         debugPrint('Found some text that unexpectedly renders at different heights.');
         debugPrint('Text: $_text');
-        debugPrint(_text.runes.map<String>((int index) => 'U+' + index.toRadixString(16).padLeft(4, '0')).join(' '));
+        debugPrint(_text?.runes.map<String>((int index) => 'U+' + index.toRadixString(16).padLeft(4, '0')).join(' '));
         setState(() {
           _ticker.stop();
         });
@@ -1080,7 +1076,7 @@ class _PaintingState extends State<Painting> with SingleTickerProviderStateMixin
                       TextButton(
                         onPressed: _ticker.isActive ? null : () {
                           print('The currently visible text is: $_text');
-                          print(_text.runes.map<String>((int value) => 'U+${value.toRadixString(16).padLeft(4, '0').toUpperCase()}').join(' '));
+                          print(_text?.runes.map<String>((int value) => 'U+${value.toRadixString(16).padLeft(4, '0').toUpperCase()}').join(' '));
                         },
                         child: const Text('DUMP TEXT TO LOGS'),
                       ),
@@ -1096,7 +1092,7 @@ class _PaintingState extends State<Painting> with SingleTickerProviderStateMixin
   }
 }
 
-String zalgo(math.Random random, int targetLength, { bool includeSpacingCombiningMarks = false, String base }) {
+String zalgo(math.Random random, int targetLength, { bool includeSpacingCombiningMarks = false, String? base }) {
   // The following three tables are derived from UnicodeData.txt:
   //   http://unicode.org/Public/UNIDATA/UnicodeData.txt
   // There are three groups, character classes Mc, Me, and Mn.
diff --git a/dev/manual_tests/pubspec.yaml b/dev/manual_tests/pubspec.yaml
index a255931d11..dcc894bdc0 100644
--- a/dev/manual_tests/pubspec.yaml
+++ b/dev/manual_tests/pubspec.yaml
@@ -1,7 +1,7 @@
 name: manual_tests
 
 environment:
-  sdk: ">=2.2.2 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter:
diff --git a/dev/manual_tests/test/mock_image_http.dart b/dev/manual_tests/test/mock_image_http.dart
index cf2ef91fdc..241a997995 100644
--- a/dev/manual_tests/test/mock_image_http.dart
+++ b/dev/manual_tests/test/mock_image_http.dart
@@ -10,7 +10,7 @@ import 'package:flutter_test/flutter_test.dart';
 import '../../../packages/flutter/test/image_data.dart';
 
 // Returns a mock HTTP client that responds with an image to all requests.
-FakeHttpClient createMockImageHttpClient(SecurityContext _) {
+FakeHttpClient createMockImageHttpClient(SecurityContext? _) {
   final FakeHttpClient client = FakeHttpClient();
   return client;
 }
@@ -50,10 +50,10 @@ class FakeHttpClientResponse extends Fake implements HttpClientResponse {
   HttpClientResponseCompressionState get compressionState => HttpClientResponseCompressionState.notCompressed;
 
   @override
-  StreamSubscription<List<int>> listen(void Function(List<int>) onData, {
-    void Function() onDone,
-    Function onError,
-    bool cancelOnError,
+  StreamSubscription<List<int>> listen(void Function(List<int>)? onData, {
+    void Function()? onDone,
+    Function? onError,
+    bool? cancelOnError,
   }) {
     return Stream<List<int>>.fromIterable(<List<int>>[kTransparentImage])
       .listen(onData, onDone: onDone, onError: onError, cancelOnError: cancelOnError);