Migrate Flutter Gallery test to null safety (#70116)

dev/benchmarks/microbenchmarks/pubspec.yaml

@@ -29,15 +29,13 @@ dependencies:
   coverage: 0.14.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   crypto: 2.1.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   csslib: 0.16.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  dart_style: 1.3.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   fake_async: 1.2.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   glob: 1.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   html: 0.14.0+4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   http: 0.12.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   http_multi_server: 2.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   http_parser: 3.1.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  intl: 0.16.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  intl_translation: 0.17.10+1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  intl: 0.17.0-nullsafety.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   io: 0.3.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   isolate: 2.0.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   js: 0.6.3-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
@@ -50,7 +48,6 @@ dependencies:
   package_config: 1.9.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   path: 1.8.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   pedantic: 1.10.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  petitparser: 3.1.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   pool: 1.5.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   pub_semver: 1.4.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   shelf: 0.7.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
@@ -77,4 +74,4 @@ dependencies:
 flutter:
   uses-material-design: true
 
-# PUBSPEC CHECKSUM: 77af
+# PUBSPEC CHECKSUM: 2c1c

dev/benchmarks/test_apps/stocks/pubspec.yaml

@@ -9,42 +9,23 @@ dependencies:
     sdk: flutter
   flutter_localizations:
     sdk: flutter
-  intl: 0.16.1
-  intl_translation: 0.17.10+1
+  intl: 0.17.0-nullsafety.2
   http: 0.12.2
   isolate: 2.0.3
 
-  _fe_analyzer_shared: 7.0.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  analyzer: 0.39.17 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  args: 1.6.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  async: 2.5.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   characters: 1.1.0-nullsafety.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   charcode: 1.2.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  cli_util: 0.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  clock: 1.1.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   collection: 1.15.0-nullsafety.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  convert: 2.1.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  crypto: 2.1.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  csslib: 0.16.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  dart_style: 1.3.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  glob: 1.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  html: 0.14.0+4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   http_parser: 3.1.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  js: 0.6.3-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   meta: 1.3.0-nullsafety.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  node_interop: 1.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  node_io: 1.1.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  package_config: 1.9.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   path: 1.8.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   pedantic: 1.10.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  petitparser: 3.1.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  pub_semver: 1.4.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   source_span: 1.8.0-nullsafety.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   string_scanner: 1.1.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   term_glyph: 1.2.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   typed_data: 1.3.0-nullsafety.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   vector_math: 2.1.0-nullsafety.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  watcher: 0.9.7+15 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  yaml: 2.2.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
 
 dev_dependencies:
   flutter_test:
@@ -53,19 +34,33 @@ dev_dependencies:
     sdk: flutter
   test: 1.16.0-nullsafety.9
 
+  _fe_analyzer_shared: 7.0.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  analyzer: 0.39.17 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   archive: 2.0.13 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  args: 1.6.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  async: 2.5.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   boolean_selector: 2.1.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  clock: 1.1.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  cli_util: 0.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  convert: 2.1.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   coverage: 0.14.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  crypto: 2.1.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  csslib: 0.16.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   fake_async: 1.2.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   file: 6.0.0-nullsafety.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  glob: 1.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  html: 0.14.0+4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   http_multi_server: 2.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   io: 0.3.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  js: 0.6.3-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   logging: 0.11.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   matcher: 0.12.10-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   mime: 0.9.7 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  node_interop: 1.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  node_io: 1.1.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   node_preamble: 1.4.12 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  package_config: 1.9.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   pool: 1.5.0-nullsafety.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  pub_semver: 1.4.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   shelf: 0.7.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   shelf_packages_handler: 2.0.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   shelf_static: 0.2.8 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
@@ -78,11 +73,13 @@ dev_dependencies:
   test_api: 0.2.19-nullsafety.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   test_core: 0.3.12-nullsafety.9 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   vm_service: 5.2.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  watcher: 0.9.7+15 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   web_socket_channel: 1.1.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   webdriver: 2.1.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   webkit_inspection_protocol: 0.7.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  yaml: 2.2.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
 
 flutter:
   uses-material-design: true
 
-# PUBSPEC CHECKSUM: b21f
+# PUBSPEC CHECKSUM: 4b8b

dev/integration_tests/flutter_gallery/lib/demo/animation/sections.dart

@@ -20,10 +20,10 @@ class SectionDetail {
     this.imageAsset,
     this.imageAssetPackage,
   });
-  final String title;
-  final String subtitle;
-  final String imageAsset;
-  final String imageAssetPackage;
+  final String? title;
+  final String? subtitle;
+  final String? imageAsset;
+  final String? imageAssetPackage;
 }
 
 @immutable
@@ -36,12 +36,12 @@ class Section {
     this.rightColor,
     this.details,
   });
-  final String title;
-  final String backgroundAsset;
-  final String backgroundAssetPackage;
-  final Color leftColor;
-  final Color rightColor;
-  final List<SectionDetail> details;
+  final String? title;
+  final String? backgroundAsset;
+  final String? backgroundAssetPackage;
+  final Color? leftColor;
+  final Color? rightColor;
+  final List<SectionDetail>? details;
 
   @override
   bool operator==(Object other) {

dev/integration_tests/flutter_gallery/lib/demo/animation/widgets.dart

@@ -10,9 +10,8 @@ const double kSectionIndicatorWidth = 32.0;
 
 // The card for a single section. Displays the section's gradient and background image.
 class SectionCard extends StatelessWidget {
-  const SectionCard({ Key key, @required this.section })
-    : assert(section != null),
-      super(key: key);
+  const SectionCard({ Key? key, required this.section })
+    : super(key: key);
 
   final Section section;
 
@@ -27,13 +26,13 @@ class SectionCard extends StatelessWidget {
             begin: Alignment.centerLeft,
             end: Alignment.centerRight,
             colors: <Color>[
-              section.leftColor,
-              section.rightColor,
+              section.leftColor!,
+              section.rightColor!,
             ],
           ),
         ),
         child: Image.asset(
-          section.backgroundAsset,
+          section.backgroundAsset!,
           package: section.backgroundAssetPackage,
           color: const Color.fromRGBO(255, 255, 255, 0.075),
           colorBlendMode: BlendMode.modulate,
@@ -48,13 +47,11 @@ class SectionCard extends StatelessWidget {
 // offset a little. It's supposed to look sort-of 3D.
 class SectionTitle extends StatelessWidget {
   const SectionTitle({
-    Key key,
-    @required this.section,
-    @required this.scale,
-    @required this.opacity,
-  }) : assert(section != null),
-       assert(scale != null),
-       assert(opacity != null && opacity >= 0.0 && opacity <= 1.0),
+    Key? key,
+    required this.section,
+    required this.scale,
+    required this.opacity,
+  }) : assert(opacity >= 0.0 && opacity <= 1.0),
        super(key: key);
 
   final Section section;
@@ -86,9 +83,9 @@ class SectionTitle extends StatelessWidget {
             children: <Widget>[
               Positioned(
                 top: 4.0,
-                child: Text(section.title, style: sectionTitleShadowStyle),
+                child: Text(section.title!, style: sectionTitleShadowStyle),
               ),
-              Text(section.title, style: sectionTitleStyle),
+              Text(section.title!, style: sectionTitleStyle),
             ],
           ),
         ),
@@ -99,7 +96,7 @@ class SectionTitle extends StatelessWidget {
 
 // Small horizontal bar that indicates the selected section.
 class SectionIndicator extends StatelessWidget {
-  const SectionIndicator({ Key key, this.opacity = 1.0 }) : super(key: key);
+  const SectionIndicator({ Key? key, this.opacity = 1.0 }) : super(key: key);
 
   final double opacity;
 
@@ -117,8 +114,8 @@ class SectionIndicator extends StatelessWidget {
 
 // Display a single SectionDetail.
 class SectionDetailView extends StatelessWidget {
-  SectionDetailView({ Key key, @required this.detail })
-    : assert(detail != null && detail.imageAsset != null),
+  SectionDetailView({ Key? key, required this.detail })
+    : assert(detail.imageAsset != null),
       assert((detail.imageAsset ?? detail.title) != null),
       super(key: key);
 
@@ -131,7 +128,7 @@ class SectionDetailView extends StatelessWidget {
         borderRadius: BorderRadius.circular(6.0),
         image: DecorationImage(
           image: AssetImage(
-            detail.imageAsset,
+            detail.imageAsset!,
             package: detail.imageAssetPackage,
           ),
           fit: BoxFit.cover,
@@ -153,8 +150,8 @@ class SectionDetailView extends StatelessWidget {
       );
     } else {
       item = ListTile(
-        title: Text(detail.title),
-        subtitle: Text(detail.subtitle),
+        title: Text(detail.title!),
+        subtitle: Text(detail.subtitle!),
         leading: SizedBox(width: 32.0, height: 32.0, child: image),
       );
     }

dev/integration_tests/flutter_gallery/lib/demo/animation_demo.dart

@@ -7,7 +7,7 @@ import 'package:flutter/material.dart';
 import 'animation/home.dart';
 
 class AnimationDemo extends StatelessWidget {
-  const AnimationDemo({Key key}) : super(key: key);
+  const AnimationDemo({Key? key}) : super(key: key);
 
   static const String routeName = '/animation';
 

dev/integration_tests/flutter_gallery/lib/demo/calculator/home.dart

@@ -7,7 +7,7 @@ import 'package:flutter/material.dart';
 import 'logic.dart';
 
 class Calculator extends StatefulWidget {
-  const Calculator({Key key}) : super(key: key);
+  const Calculator({Key? key}) : super(key: key);
 
   @override
   _CalculatorState createState() => _CalculatorState();
@@ -43,7 +43,7 @@ class _CalculatorState extends State<Calculator> {
   }
 
   void handleNumberTap(int n) {
-    final CalcExpression expression = _expression.appendDigit(n);
+    final CalcExpression? expression = _expression.appendDigit(n);
     if (expression != null) {
       setState(() {
         pushExpression(expression);
@@ -52,7 +52,7 @@ class _CalculatorState extends State<Calculator> {
   }
 
   void handlePointTap() {
-    final CalcExpression expression = _expression.appendPoint();
+    final CalcExpression? expression = _expression.appendPoint();
     if (expression != null) {
       setState(() {
         pushExpression(expression);
@@ -61,7 +61,7 @@ class _CalculatorState extends State<Calculator> {
   }
 
   void handlePlusTap() {
-    final CalcExpression expression = _expression.appendOperation(Operation.Addition);
+    final CalcExpression? expression = _expression.appendOperation(Operation.Addition);
     if (expression != null) {
       setState(() {
         pushExpression(expression);
@@ -70,7 +70,7 @@ class _CalculatorState extends State<Calculator> {
   }
 
   void handleMinusTap() {
-    final CalcExpression expression = _expression.appendMinus();
+    final CalcExpression? expression = _expression.appendMinus();
     if (expression != null) {
       setState(() {
         pushExpression(expression);
@@ -79,7 +79,7 @@ class _CalculatorState extends State<Calculator> {
   }
 
   void handleMultTap() {
-    final CalcExpression expression = _expression.appendOperation(Operation.Multiplication);
+    final CalcExpression? expression = _expression.appendOperation(Operation.Multiplication);
     if (expression != null) {
       setState(() {
         pushExpression(expression);
@@ -88,7 +88,7 @@ class _CalculatorState extends State<Calculator> {
   }
 
   void handleDivTap() {
-    final CalcExpression expression = _expression.appendOperation(Operation.Division);
+    final CalcExpression? expression = _expression.appendOperation(Operation.Division);
     if (expression != null) {
       setState(() {
         pushExpression(expression);
@@ -97,7 +97,7 @@ class _CalculatorState extends State<Calculator> {
   }
 
   void handleEqualsTap() {
-    final CalcExpression resultExpression = _expression.computeResult();
+    final CalcExpression? resultExpression = _expression.computeResult();
     if (resultExpression != null) {
       setState(() {
         setResult(resultExpression);
@@ -140,13 +140,13 @@ class _CalculatorState extends State<Calculator> {
 class CalcDisplay extends StatelessWidget {
   const CalcDisplay({ this.content });
 
-  final String content;
+  final String? content;
 
   @override
   Widget build(BuildContext context) {
     return Center(
       child: Text(
-        content,
+        content!,
         style: const TextStyle(fontSize: 24.0),
       ),
     );
@@ -156,7 +156,7 @@ class CalcDisplay extends StatelessWidget {
 class KeyPad extends StatelessWidget {
   const KeyPad({ this.calcState });
 
-  final _CalculatorState calcState;
+  final _CalculatorState? calcState;
 
   @override
   Widget build(BuildContext context) {
@@ -193,9 +193,9 @@ class KeyPad extends StatelessWidget {
                     NumberKey(3, calcState),
                   ]),
                   KeyRow(<Widget>[
-                    CalcKey('.', calcState.handlePointTap),
+                    CalcKey('.', calcState!.handlePointTap),
                     NumberKey(0, calcState),
-                    CalcKey('=', calcState.handleEqualsTap),
+                    CalcKey('=', calcState!.handleEqualsTap),
                   ]),
                 ],
               ),
@@ -205,11 +205,11 @@ class KeyPad extends StatelessWidget {
                 color: themeData.backgroundColor,
                 child: Column(
                   children: <Widget>[
-                    CalcKey('\u232B', calcState.handleDelTap),
-                    CalcKey('\u00F7', calcState.handleDivTap),
-                    CalcKey('\u00D7', calcState.handleMultTap),
-                    CalcKey('-', calcState.handleMinusTap),
-                    CalcKey('+', calcState.handlePlusTap),
+                    CalcKey('\u232B', calcState!.handleDelTap),
+                    CalcKey('\u00F7', calcState!.handleDivTap),
+                    CalcKey('\u00D7', calcState!.handleMultTap),
+                    CalcKey('-', calcState!.handleMinusTap),
+                    CalcKey('+', calcState!.handlePlusTap),
                   ],
                 ),
               ),
@@ -263,8 +263,8 @@ class CalcKey extends StatelessWidget {
 }
 
 class NumberKey extends CalcKey {
-  NumberKey(int value, _CalculatorState calcState)
+  NumberKey(int value, _CalculatorState? calcState)
     : super('$value', () {
-        calcState.handleNumberTap(value);
+        calcState!.handleNumberTap(value);
       });
 }

dev/integration_tests/flutter_gallery/lib/demo/calculator/logic.dart

@@ -9,10 +9,10 @@
 class ExpressionToken {
   ExpressionToken(this.stringRep);
 
-  final String stringRep;
+  final String? stringRep;
 
   @override
-  String toString() => stringRep;
+  String toString() => stringRep!;
 }
 
 /// A token that represents a number.
@@ -71,7 +71,7 @@ class OperationToken extends ExpressionToken {
 
   Operation operation;
 
-  static String opString(Operation operation) {
+  static String? opString(Operation operation) {
     switch (operation) {
       case Operation.Addition:
         return ' + ';
@@ -82,8 +82,6 @@ class OperationToken extends ExpressionToken {
       case Operation.Division:
         return '  \u00F7  ';
     }
-    assert(operation != null);
-    return null;
   }
 }
 
@@ -124,13 +122,13 @@ class CalcExpression {
     : this(<ExpressionToken>[], ExpressionState.Start);
 
   CalcExpression.result(FloatToken result)
-    : _list = <ExpressionToken>[],
+    : _list = <ExpressionToken?>[],
       state = ExpressionState.Result {
     _list.add(result);
   }
 
   /// The tokens comprising the expression.
-  final List<ExpressionToken> _list;
+  final List<ExpressionToken?> _list;
   /// The state of the expression.
   final ExpressionState state;
 
@@ -146,10 +144,10 @@ class CalcExpression {
   /// Append a digit to the current expression and return a new expression
   /// representing the result. Returns null to indicate that it is not legal
   /// to append a digit in the current state.
-  CalcExpression appendDigit(int digit) {
+  CalcExpression? appendDigit(int digit) {
     ExpressionState newState = ExpressionState.Number;
-    ExpressionToken newToken;
-    final List<ExpressionToken> outList = _list.toList();
+    ExpressionToken? newToken;
+    final List<ExpressionToken?> outList = _list.toList();
     switch (state) {
       case ExpressionState.Start:
         // Start a new number with digit.
@@ -161,12 +159,12 @@ class CalcExpression {
         newToken = IntToken('-$digit');
         break;
       case ExpressionState.Number:
-        final ExpressionToken last = outList.removeLast();
+        final ExpressionToken last = outList.removeLast()!;
         newToken = IntToken('${last.stringRep}$digit');
         break;
       case ExpressionState.Point:
       case ExpressionState.NumberWithPoint:
-        final ExpressionToken last = outList.removeLast();
+        final ExpressionToken last = outList.removeLast()!;
         newState = ExpressionState.NumberWithPoint;
         newToken = FloatToken('${last.stringRep}$digit');
         break;
@@ -181,17 +179,17 @@ class CalcExpression {
   /// Append a point to the current expression and return a new expression
   /// representing the result. Returns null to indicate that it is not legal
   /// to append a point in the current state.
-  CalcExpression appendPoint() {
-    ExpressionToken newToken;
-    final List<ExpressionToken> outList = _list.toList();
+  CalcExpression? appendPoint() {
+    ExpressionToken? newToken;
+    final List<ExpressionToken?> outList = _list.toList();
     switch (state) {
       case ExpressionState.Start:
         newToken = FloatToken('.');
         break;
       case ExpressionState.LeadingNeg:
       case ExpressionState.Number:
-        final ExpressionToken last = outList.removeLast();
-        newToken = FloatToken(last.stringRep + '.');
+        final ExpressionToken last = outList.removeLast()!;
+        newToken = FloatToken(last.stringRep! + '.');
         break;
       case ExpressionState.Point:
       case ExpressionState.NumberWithPoint:
@@ -206,7 +204,7 @@ class CalcExpression {
   /// Append an operation symbol to the current expression and return a new
   /// expression representing the result. Returns null to indicate that it is not
   /// legal to append an operation symbol in the current state.
-  CalcExpression appendOperation(Operation op) {
+  CalcExpression? appendOperation(Operation op) {
     switch (state) {
       case ExpressionState.Start:
       case ExpressionState.LeadingNeg:
@@ -218,7 +216,7 @@ class CalcExpression {
       case ExpressionState.Result:
         break;
     }
-    final List<ExpressionToken> outList = _list.toList();
+    final List<ExpressionToken?> outList = _list.toList();
     outList.add(OperationToken(op));
     return CalcExpression(outList, ExpressionState.Start);
   }
@@ -226,7 +224,7 @@ class CalcExpression {
   /// Append a leading minus sign to the current expression and return a new
   /// expression representing the result. Returns null to indicate that it is not
   /// legal to append a leading minus sign in the current state.
-  CalcExpression appendLeadingNeg() {
+  CalcExpression? appendLeadingNeg() {
     switch (state) {
       case ExpressionState.Start:
         break;
@@ -238,7 +236,7 @@ class CalcExpression {
         // Cannot enter leading neg now.
         return null;
     }
-    final List<ExpressionToken> outList = _list.toList();
+    final List<ExpressionToken?> outList = _list.toList();
     outList.add(LeadingNegToken());
     return CalcExpression(outList, ExpressionState.LeadingNeg);
   }
@@ -248,7 +246,7 @@ class CalcExpression {
   /// to append a minus sign in the current state. Depending on the current
   /// state the minus sign will be interpreted as either a leading negative
   /// sign or a subtraction operation.
-  CalcExpression appendMinus() {
+  CalcExpression? appendMinus() {
     switch (state) {
       case ExpressionState.Start:
         return appendLeadingNeg();
@@ -266,7 +264,7 @@ class CalcExpression {
   /// Computes the result of the current expression and returns a new
   /// ResultExpression containing the result. Returns null to indicate that
   /// it is not legal to compute a result in the current state.
-  CalcExpression computeResult() {
+  CalcExpression? computeResult() {
     switch (state) {
       case ExpressionState.Start:
       case ExpressionState.LeadingNeg:
@@ -281,13 +279,13 @@ class CalcExpression {
 
     // We make a copy of _list because CalcExpressions are supposed to
     // be immutable.
-    final List<ExpressionToken> list = _list.toList();
+    final List<ExpressionToken?> list = _list.toList();
     // We obey order-of-operations by computing the sum of the 'terms',
     // where a "term" is defined to be a sequence of numbers separated by
     // multiplication or division symbols.
     num currentTermValue = removeNextTerm(list);
     while (list.isNotEmpty) {
-      final OperationToken opToken = list.removeAt(0) as OperationToken;
+      final OperationToken opToken = list.removeAt(0)! as OperationToken;
       final num nextTermValue = removeNextTerm(list);
       switch (opToken.operation) {
         case Operation.Addition:
@@ -311,13 +309,13 @@ class CalcExpression {
   /// Removes the next "term" from `list` and returns its numeric value.
   /// A "term" is a sequence of number tokens separated by multiplication
   /// and division symbols.
-  static num removeNextTerm(List<ExpressionToken> list) {
-    assert(list != null && list.isNotEmpty);
-    final NumberToken firstNumToken = list.removeAt(0) as NumberToken;
+  static num removeNextTerm(List<ExpressionToken?> list) {
+    assert(list.isNotEmpty);
+    final NumberToken firstNumToken = list.removeAt(0)! as NumberToken;
     num currentValue = firstNumToken.number;
     while (list.isNotEmpty) {
       bool isDivision = false;
-      final OperationToken nextOpToken = list.first as OperationToken;
+      final OperationToken nextOpToken = list.first! as OperationToken;
       switch (nextOpToken.operation) {
         case Operation.Addition:
         case Operation.Subtraction:
@@ -331,7 +329,7 @@ class CalcExpression {
       // Remove the operation token.
       list.removeAt(0);
       // Remove the next number token.
-      final NumberToken nextNumToken = list.removeAt(0) as NumberToken;
+      final NumberToken nextNumToken = list.removeAt(0)! as NumberToken;
       final num nextNumber = nextNumToken.number;
       if (isDivision)
         currentValue /= nextNumber;

dev/integration_tests/flutter_gallery/lib/demo/calculator_demo.dart

@@ -7,7 +7,7 @@ import 'package:flutter/material.dart';
 import 'calculator/home.dart';
 
 class CalculatorDemo extends StatelessWidget {
-  const CalculatorDemo({Key key}) : super(key: key);
+  const CalculatorDemo({Key? key}) : super(key: key);
 
   static const String routeName = '/calculator';
 

dev/integration_tests/flutter_gallery/lib/demo/colors_demo.dart

@@ -9,12 +9,12 @@ const double kColorItemHeight = 48.0;
 class Palette {
   Palette({ this.name, this.primary, this.accent, this.threshold = 900});
 
-  final String name;
-  final MaterialColor primary;
-  final MaterialAccentColor accent;
+  final String? name;
+  final MaterialColor? primary;
+  final MaterialAccentColor? accent;
   final int threshold; // titles for indices > threshold are white, otherwise black
 
-  bool get isValid => name != null && primary != null && threshold != null;
+  bool get isValid => name != null && primary != null;
 }
 
 final List<Palette> allPalettes = <Palette>[
@@ -42,14 +42,11 @@ final List<Palette> allPalettes = <Palette>[
 
 class ColorItem extends StatelessWidget {
   const ColorItem({
-    Key key,
-    @required this.index,
-    @required this.color,
+    Key? key,
+    required this.index,
+    required this.color,
     this.prefix = '',
-  }) : assert(index != null),
-       assert(color != null),
-       assert(prefix != null),
-       super(key: key);
+  }) : super(key: key);
 
   final int index;
   final Color color;
@@ -84,9 +81,9 @@ class ColorItem extends StatelessWidget {
 
 class PaletteTabView extends StatelessWidget {
   PaletteTabView({
-    Key key,
-    @required this.colors,
-  }) : assert(colors != null && colors.isValid),
+    Key? key,
+    required this.colors,
+  }) : assert(colors.isValid),
        super(key: key);
 
   final Palette colors;
@@ -97,8 +94,8 @@ class PaletteTabView extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final TextTheme textTheme = Theme.of(context).textTheme;
-    final TextStyle whiteTextStyle = textTheme.bodyText2.copyWith(color: Colors.white);
-    final TextStyle blackTextStyle = textTheme.bodyText2.copyWith(color: Colors.black);
+    final TextStyle whiteTextStyle = textTheme.bodyText2!.copyWith(color: Colors.white);
+    final TextStyle blackTextStyle = textTheme.bodyText2!.copyWith(color: Colors.black);
     return Scrollbar(
       child: ListView(
         itemExtent: kColorItemHeight,
@@ -106,14 +103,14 @@ class PaletteTabView extends StatelessWidget {
           ...primaryKeys.map<Widget>((int index) {
             return DefaultTextStyle(
               style: index > colors.threshold ? whiteTextStyle : blackTextStyle,
-              child: ColorItem(index: index, color: colors.primary[index]),
+              child: ColorItem(index: index, color: colors.primary![index]!),
             );
           }),
           if (colors.accent != null)
             ...accentKeys.map<Widget>((int index) {
               return DefaultTextStyle(
                 style: index > colors.threshold ? whiteTextStyle : blackTextStyle,
-                child: ColorItem(index: index, color: colors.accent[index], prefix: 'A'),
+                child: ColorItem(index: index, color: colors.accent![index]!, prefix: 'A'),
               );
             }),
         ],

dev/integration_tests/flutter_gallery/lib/demo/contacts_demo.dart

@@ -6,10 +6,10 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
 class _ContactCategory extends StatelessWidget {
-  const _ContactCategory({ Key key, this.icon, this.children }) : super(key: key);
+  const _ContactCategory({ Key? key, this.icon, this.children }) : super(key: key);
 
-  final IconData icon;
-  final List<Widget> children;
+  final IconData? icon;
+  final List<Widget>? children;
 
   @override
   Widget build(BuildContext context) {
@@ -20,7 +20,7 @@ class _ContactCategory extends StatelessWidget {
         border: Border(bottom: BorderSide(color: themeData.dividerColor))
       ),
       child: DefaultTextStyle(
-        style: Theme.of(context).textTheme.subtitle1,
+        style: Theme.of(context).textTheme.subtitle1!,
         child: SafeArea(
           top: false,
           bottom: false,
@@ -32,7 +32,7 @@ class _ContactCategory extends StatelessWidget {
                 width: 72.0,
                 child: Icon(icon, color: themeData.primaryColor),
               ),
-              Expanded(child: Column(children: children)),
+              Expanded(child: Column(children: children!)),
             ],
           ),
         ),
@@ -42,14 +42,14 @@ class _ContactCategory extends StatelessWidget {
 }
 
 class _ContactItem extends StatelessWidget {
-  const _ContactItem({ Key key, this.icon, this.lines, this.tooltip, this.onPressed })
+  const _ContactItem({ Key? key, this.icon, required this.lines, this.tooltip, this.onPressed })
     : assert(lines.length > 1),
       super(key: key);
 
-  final IconData icon;
+  final IconData? icon;
   final List<String> lines;
-  final String tooltip;
-  final VoidCallback onPressed;
+  final String? tooltip;
+  final VoidCallback? onPressed;
 
   @override
   Widget build(BuildContext context) {

dev/integration_tests/flutter_gallery/lib/demo/cupertino/cupertino_alert_demo.dart

@@ -14,24 +14,24 @@ class CupertinoAlertDemo extends StatefulWidget {
 }
 
 class _CupertinoAlertDemoState extends State<CupertinoAlertDemo> {
-  String lastSelectedValue;
+  String? lastSelectedValue;
 
-  void showDemoDialog({BuildContext context, Widget child}) {
+  void showDemoDialog({required BuildContext context, Widget? child}) {
     showCupertinoDialog<String>(
       context: context,
-      builder: (BuildContext context) => child,
-    ).then((String value) {
+      builder: (BuildContext context) => child!,
+    ).then((String? value) {
       if (value != null) {
         setState(() { lastSelectedValue = value; });
       }
     });
   }
 
-  void showDemoActionSheet({BuildContext context, Widget child}) {
+  void showDemoActionSheet({required BuildContext context, Widget? child}) {
     showCupertinoModalPopup<String>(
       context: context,
-      builder: (BuildContext context) => child,
-    ).then((String value) {
+      builder: (BuildContext context) => child!,
+    ).then((String? value) {
       if (value != null) {
         setState(() { lastSelectedValue = value; });
       }
@@ -196,10 +196,10 @@ class _CupertinoAlertDemoState extends State<CupertinoAlertDemo> {
 }
 
 class CupertinoDessertDialog extends StatelessWidget {
-  const CupertinoDessertDialog({Key key, this.title, this.content}) : super(key: key);
+  const CupertinoDessertDialog({Key? key, this.title, this.content}) : super(key: key);
 
-  final Widget title;
-  final Widget content;
+  final Widget? title;
+  final Widget? content;
 
   @override
   Widget build(BuildContext context) {

dev/integration_tests/flutter_gallery/lib/demo/cupertino/cupertino_navigation_demo.dart

@@ -44,7 +44,7 @@ class CupertinoNavigationDemo extends StatelessWidget {
 
   final List<Color> colorItems;
   final List<String> colorNameItems;
-  final int randomSeed;
+  final int? randomSeed;
 
   @override
   Widget build(BuildContext context) {
@@ -71,7 +71,6 @@ class CupertinoNavigationDemo extends StatelessWidget {
             ],
           ),
           tabBuilder: (BuildContext context, int index) {
-            assert(index >= 0 && index <= 2);
             switch (index) {
               case 0:
                 return CupertinoTabView(
@@ -84,21 +83,19 @@ class CupertinoNavigationDemo extends StatelessWidget {
                   },
                   defaultTitle: 'Colors',
                 );
-                break;
               case 1:
                 return CupertinoTabView(
                   builder: (BuildContext context) => CupertinoDemoTab2(),
                   defaultTitle: 'Support Chat',
                 );
-                break;
               case 2:
                 return CupertinoTabView(
                   builder: (BuildContext context) => CupertinoDemoTab3(),
                   defaultTitle: 'Account',
                 );
-                break;
             }
-            return null;
+            assert(false);
+            return const CupertinoTabView();
           },
         ),
       ),
@@ -120,7 +117,7 @@ class ExitButton extends StatelessWidget {
       ),
       onPressed: () {
         // The demo is on the root navigator.
-        Navigator.of(context, rootNavigator: true).pop();
+        Navigator.of(context, rootNavigator: true)!.pop();
       },
     );
   }
@@ -142,9 +139,9 @@ class CupertinoDemoTab1 extends StatelessWidget {
     this.randomSeed,
   });
 
-  final List<Color> colorItems;
-  final List<String> colorNameItems;
-  final int randomSeed;
+  final List<Color>? colorItems;
+  final List<String>? colorNameItems;
+  final int? randomSeed;
 
   @override
   Widget build(BuildContext context) {
@@ -170,8 +167,8 @@ class CupertinoDemoTab1 extends StatelessWidget {
                   return Tab1RowItem(
                     index: index,
                     lastItem: index == _kChildCount - 1,
-                    color: colorItems[index],
-                    colorName: colorNameItems[index],
+                    color: colorItems![index],
+                    colorName: colorNameItems![index],
                     randomSeed: randomSeed,
                   );
                 },
@@ -194,18 +191,18 @@ class Tab1RowItem extends StatelessWidget {
     this.randomSeed,
   });
 
-  final int index;
-  final bool lastItem;
-  final Color color;
-  final String colorName;
-  final int randomSeed;
+  final int? index;
+  final bool? lastItem;
+  final Color? color;
+  final String? colorName;
+  final int? randomSeed;
 
   @override
   Widget build(BuildContext context) {
     final Widget row = GestureDetector(
       behavior: HitTestBehavior.opaque,
       onTap: () {
-        Navigator.of(context).push(CupertinoPageRoute<void>(
+        Navigator.of(context)!.push(CupertinoPageRoute<void>(
           title: colorName,
           builder: (BuildContext context) => Tab1ItemPage(
             color: color,
@@ -238,7 +235,7 @@ class Tab1RowItem extends StatelessWidget {
                     child: Column(
                       crossAxisAlignment: CrossAxisAlignment.start,
                       children: <Widget>[
-                        Text(colorName),
+                        Text(colorName!),
                         const Padding(padding: EdgeInsets.only(top: 8.0)),
                         Text(
                           'Buy this cool color',
@@ -273,7 +270,7 @@ class Tab1RowItem extends StatelessWidget {
       ),
     );
 
-    if (lastItem) {
+    if (lastItem!) {
       return row;
     }
 
@@ -292,10 +289,10 @@ class Tab1RowItem extends StatelessWidget {
 class Tab1ItemPage extends StatefulWidget {
   const Tab1ItemPage({this.color, this.colorName, this.index, this.randomSeed});
 
-  final Color color;
-  final String colorName;
-  final int index;
-  final int randomSeed;
+  final Color? color;
+  final String? colorName;
+  final int? index;
+  final int? randomSeed;
 
   @override
   State<StatefulWidget> createState() => Tab1ItemPageState();
@@ -309,14 +306,14 @@ class Tab1ItemPageState extends State<Tab1ItemPage> {
       final math.Random random = math.Random(widget.randomSeed);
       return Color.fromARGB(
         255,
-        (widget.color.red + random.nextInt(100) - 50).clamp(0, 255) as int,
-        (widget.color.green + random.nextInt(100) - 50).clamp(0, 255) as int,
-        (widget.color.blue + random.nextInt(100) - 50).clamp(0, 255) as int,
+        (widget.color!.red + random.nextInt(100) - 50).clamp(0, 255),
+        (widget.color!.green + random.nextInt(100) - 50).clamp(0, 255),
+        (widget.color!.blue + random.nextInt(100) - 50).clamp(0, 255),
       );
     });
   }
 
-  List<Color> relatedColors;
+  late List<Color> relatedColors;
 
   @override
   Widget build(BuildContext context) {
@@ -350,7 +347,7 @@ class Tab1ItemPageState extends State<Tab1ItemPage> {
                       mainAxisSize: MainAxisSize.min,
                       children: <Widget>[
                         Text(
-                          widget.colorName,
+                          widget.colorName!,
                           style: const TextStyle(fontSize: 24.0, fontWeight: FontWeight.bold),
                         ),
                         const Padding(padding: EdgeInsets.only(top: 6.0)),
@@ -594,13 +591,13 @@ enum Tab2ConversationBubbleColor {
 class Tab2ConversationBubble extends StatelessWidget {
   const Tab2ConversationBubble({this.text, this.color});
 
-  final String text;
-  final Tab2ConversationBubbleColor color;
+  final String? text;
+  final Tab2ConversationBubbleColor? color;
 
   @override
   Widget build(BuildContext context) {
-    Color backgroundColor;
-    Color foregroundColor;
+    Color? backgroundColor;
+    Color? foregroundColor;
 
     switch (color) {
       case Tab2ConversationBubbleColor.gray:
@@ -611,6 +608,8 @@ class Tab2ConversationBubble extends StatelessWidget {
         backgroundColor = CupertinoTheme.of(context).primaryColor;
         foregroundColor = CupertinoColors.white;
         break;
+      default:
+        break;
     }
 
     return Container(
@@ -621,7 +620,7 @@ class Tab2ConversationBubble extends StatelessWidget {
       margin: const EdgeInsets.symmetric(horizontal: 8.0, vertical: 8.0),
       padding: const EdgeInsets.symmetric(horizontal: 14.0, vertical: 10.0),
       child: Text(
-        text,
+        text!,
         style: TextStyle(
           color: foregroundColor,
           letterSpacing: -0.4,
@@ -636,8 +635,8 @@ class Tab2ConversationBubble extends StatelessWidget {
 class Tab2ConversationAvatar extends StatelessWidget {
   const Tab2ConversationAvatar({this.text, this.color});
 
-  final String text;
-  final Color color;
+  final String? text;
+  final Color? color;
 
   @override
   Widget build(BuildContext context) {
@@ -648,20 +647,20 @@ class Tab2ConversationAvatar extends StatelessWidget {
           begin: FractionalOffset.topCenter,
           end: FractionalOffset.bottomCenter,
           colors: <Color>[
-            color,
+            color!,
             Color.fromARGB(
-              color.alpha,
-              (color.red - 60).clamp(0, 255) as int,
-              (color.green - 60).clamp(0, 255) as int,
-              (color.blue - 60).clamp(0, 255) as int,
+              color!.alpha,
+              (color!.red - 60).clamp(0, 255),
+              (color!.green - 60).clamp(0, 255),
+              (color!.blue - 60).clamp(0, 255),
             ),
-          ],
+          ]
         ),
       ),
       margin: const EdgeInsets.only(left: 8.0, bottom: 8.0),
       padding: const EdgeInsets.all(12.0),
       child: Text(
-        text,
+        text!,
         style: const TextStyle(
           color: CupertinoColors.white,
           fontSize: 13.0,
@@ -675,8 +674,8 @@ class Tab2ConversationAvatar extends StatelessWidget {
 class Tab2ConversationRow extends StatelessWidget {
   const Tab2ConversationRow({this.avatar, this.text});
 
-  final Tab2ConversationAvatar avatar;
-  final String text;
+  final Tab2ConversationAvatar? avatar;
+  final String? text;
 
   @override
   Widget build(BuildContext context) {
@@ -687,7 +686,8 @@ class Tab2ConversationRow extends StatelessWidget {
         mainAxisSize: MainAxisSize.min,
         crossAxisAlignment: isSelf ? CrossAxisAlignment.center : CrossAxisAlignment.end,
         children: <Widget>[
-          if (avatar != null) avatar,
+          if (avatar != null)
+            avatar!,
           CupertinoUserInterfaceLevel(
             data: CupertinoUserInterfaceLevelData.elevated,
             child: Tab2ConversationBubble(
@@ -752,7 +752,7 @@ class CupertinoDemoTab3 extends StatelessWidget {
           const Padding(padding: EdgeInsets.only(top: 32.0)),
           GestureDetector(
             onTap: () {
-              Navigator.of(context, rootNavigator: true).push(
+              Navigator.of(context, rootNavigator: true)!.push(
                 CupertinoPageRoute<bool>(
                   fullscreenDialog: true,
                   builder: (BuildContext context) => Tab3Dialog(),
@@ -800,7 +800,7 @@ class Tab3Dialog extends StatelessWidget {
           child: const Text('Cancel'),
           padding: EdgeInsets.zero,
           onPressed: () {
-            Navigator.of(context).pop(false);
+            Navigator.of(context)!.pop(false);
           },
         ),
       ),

dev/integration_tests/flutter_gallery/lib/demo/cupertino/cupertino_picker_demo.dart

@@ -20,10 +20,9 @@ class CupertinoPickerDemo extends StatefulWidget {
 
 class _BottomPicker extends StatelessWidget {
   const _BottomPicker({
-    Key key,
-    @required this.child,
-  }) : assert(child != null),
-       super(key: key);
+    Key? key,
+    required this.child,
+  }) : super(key: key);
 
   final Widget child;
 
@@ -53,10 +52,9 @@ class _BottomPicker extends StatelessWidget {
 
 class _Menu extends StatelessWidget {
   const _Menu({
-    Key key,
-    @required this.children,
-  }) : assert(children != null),
-       super(key: key);
+    Key? key,
+    required this.children,
+  }) : super(key: key);
 
   final List<Widget> children;
 

dev/integration_tests/flutter_gallery/lib/demo/cupertino/cupertino_refresh_demo.dart

@@ -16,7 +16,7 @@ class CupertinoRefreshControlDemo extends StatefulWidget {
 }
 
 class _CupertinoRefreshControlDemoState extends State<CupertinoRefreshControlDemo> {
-  List<List<String>> randomizedContacts;
+  late List<List<String>> randomizedContacts;
 
   @override
   void initState() {
@@ -64,7 +64,7 @@ class _CupertinoRefreshControlDemoState extends State<CupertinoRefreshControlDem
             CupertinoSliverRefreshControl(
               onRefresh: () {
                 return Future<void>.delayed(const Duration(seconds: 2))
-                ..then<void>((_) {
+                ..then((_) {
                     if (mounted) {
                       setState(() => repopulateList());
                     }
@@ -149,10 +149,10 @@ class _ListItem extends StatelessWidget {
     this.called,
   });
 
-  final String name;
-  final String place;
-  final String date;
-  final bool called;
+  final String? name;
+  final String? place;
+  final String? date;
+  final bool? called;
 
   @override
   Widget build(BuildContext context) {
@@ -164,7 +164,7 @@ class _ListItem extends StatelessWidget {
         children: <Widget>[
           Container(
             width: 38.0,
-            child: called
+            child: called!
                 ? Align(
                     alignment: Alignment.topCenter,
                     child: Icon(
@@ -191,7 +191,7 @@ class _ListItem extends StatelessWidget {
                       mainAxisAlignment: MainAxisAlignment.spaceBetween,
                       children: <Widget>[
                         Text(
-                          name,
+                          name!,
                           maxLines: 1,
                           overflow: TextOverflow.ellipsis,
                           style: const TextStyle(
@@ -200,7 +200,7 @@ class _ListItem extends StatelessWidget {
                           ),
                         ),
                         Text(
-                          place,
+                          place!,
                           maxLines: 1,
                           overflow: TextOverflow.ellipsis,
                           style: TextStyle(
@@ -213,7 +213,7 @@ class _ListItem extends StatelessWidget {
                     ),
                   ),
                   Text(
-                    date,
+                    date!,
                     style: TextStyle(
                       color: CupertinoColors.inactiveGray.resolveFrom(context),
                       fontSize: 15.0,

dev/integration_tests/flutter_gallery/lib/demo/cupertino/cupertino_segmented_control_demo.dart

@@ -43,9 +43,9 @@ class _CupertinoSegmentedControlDemoState extends State<CupertinoSegmentedContro
     ),
   };
 
-  int currentSegment = 0;
+  int? currentSegment = 0;
 
-  void onValueChanged(int newValue) {
+  void onValueChanged(int? newValue) {
     setState(() {
       currentSegment = newValue;
     });
@@ -126,7 +126,7 @@ class _CupertinoSegmentedControlDemoState extends State<CupertinoSegmentedContro
                               ),
                             ],
                           ),
-                          child: icons[currentSegment],
+                          child: icons[currentSegment!],
                         );
                       },
                     ),

dev/integration_tests/flutter_gallery/lib/demo/cupertino/cupertino_text_field_demo.dart

@@ -14,8 +14,8 @@ class CupertinoTextFieldDemo extends StatefulWidget {
 }
 
 class _CupertinoTextFieldDemoState extends State<CupertinoTextFieldDemo> {
-  TextEditingController _chatTextController;
-  TextEditingController _locationTextController;
+  TextEditingController? _chatTextController;
+  TextEditingController? _locationTextController;
 
   @override
   void initState() {
@@ -49,12 +49,12 @@ class _CupertinoTextFieldDemoState extends State<CupertinoTextFieldDemo> {
             color: CupertinoColors.activeGreen,
           ),
           padding: const EdgeInsets.only(bottom: 4),
-          onPressed: ()=> setState(()=> _chatTextController.clear()),
+          onPressed: ()=> setState(()=> _chatTextController!.clear()),
         ),
       ),
       autofocus: true,
       suffixMode: OverlayVisibilityMode.editing,
-      onSubmitted: (String text)=> setState(()=> _chatTextController.clear()),
+      onSubmitted: (String text)=> setState(()=> _chatTextController!.clear()),
     );
   }
 

dev/integration_tests/flutter_gallery/lib/demo/fortnightly/fortnightly.dart

@@ -33,7 +33,7 @@ class FortnightlyDemo extends StatelessWidget {
 class ShortAppBar extends StatelessWidget {
   const ShortAppBar({ this.onBackPressed });
 
-  final VoidCallback onBackPressed;
+  final VoidCallback? onBackPressed;
 
   @override
   Widget build(BuildContext context) {
@@ -118,7 +118,7 @@ over water meant for the whole central valley of California? The story will shoc
                         Text(
                           ' ¬ ',
                           // TODO(larche): Replace textTheme.headline2.color with a ColorScheme value when known.
-                          style: textTheme.overline.apply(color: textTheme.headline2.color),
+                          style: textTheme.overline!.apply(color: textTheme.headline2!.color),
                         ),
                         Text(
                           'CULTURE',
@@ -196,7 +196,7 @@ TextTheme _buildTextTheme(TextTheme base) {
   theme = theme.apply(displayColor: Colors.black);
 
   theme = theme.copyWith(
-    headline4: base.headline4.copyWith(
+    headline4: base.headline4!.copyWith(
       fontFamily: 'Merriweather',
       fontStyle: FontStyle.italic,
       fontSize: 28,
@@ -204,21 +204,21 @@ TextTheme _buildTextTheme(TextTheme base) {
       color: Colors.black,
       height: .88,
     ),
-    headline2: base.headline2.copyWith(
+    headline2: base.headline2!.copyWith(
       fontFamily: 'LibreFranklin',
       fontSize: 18,
       fontWeight: FontWeight.w500,
       color: Colors.black.withAlpha(153),
     ),
-    headline5: base.headline5.copyWith(fontWeight: FontWeight.w500),
-    bodyText2: base.bodyText2.copyWith(
+    headline5: base.headline5!.copyWith(fontWeight: FontWeight.w500),
+    bodyText2: base.bodyText2!.copyWith(
       fontFamily: 'Merriweather',
       fontSize: 14,
       fontWeight: FontWeight.w300,
       color: const Color(0xFF666666),
       height: 1.11,
     ),
-    bodyText1: base.bodyText1.copyWith(
+    bodyText1: base.bodyText1!.copyWith(
       fontFamily: 'Merriweather',
       fontSize: 16,
       fontWeight: FontWeight.w300,

dev/integration_tests/flutter_gallery/lib/demo/material/backdrop_demo.dart

@@ -12,8 +12,8 @@ import 'package:flutter/material.dart';
 
 class Category {
   const Category({ this.title, this.assets });
-  final String title;
-  final List<String> assets;
+  final String? title;
+  final List<String>? assets;
   @override
   String toString() => '$runtimeType("$title")';
 }
@@ -95,21 +95,21 @@ const List<Category> allCategories = <Category>[
 ];
 
 class CategoryView extends StatelessWidget {
-  const CategoryView({ Key key, this.category }) : super(key: key);
+  const CategoryView({ Key? key, this.category }) : super(key: key);
 
-  final Category category;
+  final Category? category;
 
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
     return Scrollbar(
       child: ListView(
-        key: PageStorageKey<Category>(category),
+        key: PageStorageKey<Category?>(category),
         padding: const EdgeInsets.symmetric(
           vertical: 16.0,
           horizontal: 64.0,
         ),
-        children: category.assets.map<Widget>((String asset) {
+        children: category!.assets!.map<Widget>((String asset) {
           return Column(
             crossAxisAlignment: CrossAxisAlignment.stretch,
             children: <Widget>[
@@ -149,7 +149,7 @@ class CategoryView extends StatelessWidget {
 // BackdropDemo.
 class BackdropPanel extends StatelessWidget {
   const BackdropPanel({
-    Key key,
+    Key? key,
     this.onTap,
     this.onVerticalDragUpdate,
     this.onVerticalDragEnd,
@@ -157,11 +157,11 @@ class BackdropPanel extends StatelessWidget {
     this.child,
   }) : super(key: key);
 
-  final VoidCallback onTap;
-  final GestureDragUpdateCallback onVerticalDragUpdate;
-  final GestureDragEndCallback onVerticalDragEnd;
-  final Widget title;
-  final Widget child;
+  final VoidCallback? onTap;
+  final GestureDragUpdateCallback? onVerticalDragUpdate;
+  final GestureDragEndCallback? onVerticalDragEnd;
+  final Widget? title;
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {
@@ -185,7 +185,7 @@ class BackdropPanel extends StatelessWidget {
               padding: const EdgeInsetsDirectional.only(start: 16.0),
               alignment: AlignmentDirectional.centerStart,
               child: DefaultTextStyle(
-                style: theme.textTheme.subtitle1,
+                style: theme.textTheme.subtitle1!,
                 child: Tooltip(
                   message: 'Tap to dismiss',
                   child: title,
@@ -194,7 +194,7 @@ class BackdropPanel extends StatelessWidget {
             ),
           ),
           const Divider(height: 1.0),
-          Expanded(child: child),
+          Expanded(child: child!),
         ],
       ),
     );
@@ -204,15 +204,15 @@ class BackdropPanel extends StatelessWidget {
 // Cross fades between 'Select a Category' and 'Asset Viewer'.
 class BackdropTitle extends AnimatedWidget {
   const BackdropTitle({
-    Key key,
-    Animation<double> listenable,
+    Key? key,
+    required Animation<double> listenable,
   }) : super(key: key, listenable: listenable);
 
   @override
   Widget build(BuildContext context) {
     final Animation<double> animation = listenable as Animation<double>;
     return DefaultTextStyle(
-      style: Theme.of(context).primaryTextTheme.headline6,
+      style: Theme.of(context).primaryTextTheme.headline6!,
       softWrap: false,
       overflow: TextOverflow.ellipsis,
       child: Stack(
@@ -247,7 +247,7 @@ class BackdropDemo extends StatefulWidget {
 
 class _BackdropDemoState extends State<BackdropDemo> with SingleTickerProviderStateMixin {
   final GlobalKey _backdropKey = GlobalKey(debugLabel: 'Backdrop');
-  AnimationController _controller;
+  late AnimationController _controller;
   Category _category = allCategories[0];
 
   @override
@@ -283,7 +283,7 @@ class _BackdropDemoState extends State<BackdropDemo> with SingleTickerProviderSt
   }
 
   double get _backdropHeight {
-    final RenderBox renderBox = _backdropKey.currentContext.findRenderObject() as RenderBox;
+    final RenderBox renderBox = _backdropKey.currentContext!.findRenderObject()! as RenderBox;
     return renderBox.size.height;
   }
 
@@ -294,7 +294,7 @@ class _BackdropDemoState extends State<BackdropDemo> with SingleTickerProviderSt
     if (_controller.isAnimating || _controller.status == AnimationStatus.completed)
       return;
 
-    _controller.value -= details.primaryDelta / (_backdropHeight ?? details.primaryDelta);
+    _controller.value -= details.primaryDelta! / _backdropHeight;
   }
 
   void _handleDragEnd(DragEndDetails details) {
@@ -343,7 +343,7 @@ class _BackdropDemoState extends State<BackdropDemo> with SingleTickerProviderSt
           ? Colors.white.withOpacity(0.25)
           : Colors.transparent,
         child: ListTile(
-          title: Text(category.title),
+          title: Text(category.title!),
           selected: selected,
           onTap: () {
             _changeCategory(category);
@@ -359,8 +359,8 @@ class _BackdropDemoState extends State<BackdropDemo> with SingleTickerProviderSt
         children: <Widget>[
           ListTileTheme(
             iconColor: theme.primaryIconTheme.color,
-            textColor: theme.primaryTextTheme.headline6.color.withOpacity(0.6),
-            selectedColor: theme.primaryTextTheme.headline6.color,
+            textColor: theme.primaryTextTheme.headline6!.color!.withOpacity(0.6),
+            selectedColor: theme.primaryTextTheme.headline6!.color,
             child: Padding(
               padding: const EdgeInsets.symmetric(horizontal: 16.0),
               child: Column(
@@ -375,7 +375,7 @@ class _BackdropDemoState extends State<BackdropDemo> with SingleTickerProviderSt
               onTap: _toggleBackdropPanelVisibility,
               onVerticalDragUpdate: _handleDragUpdate,
               onVerticalDragEnd: _handleDragEnd,
-              title: Text(_category.title),
+              title: Text(_category.title!),
               child: CategoryView(category: _category),
             ),
           ),

dev/integration_tests/flutter_gallery/lib/demo/material/banner_demo.dart

@@ -13,7 +13,7 @@ enum BannerDemoAction {
 }
 
 class BannerDemo extends StatefulWidget {
-  const BannerDemo({ Key key }) : super(key: key);
+  const BannerDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/banner';
 

dev/integration_tests/flutter_gallery/lib/demo/material/bottom_app_bar_demo.dart

@@ -92,7 +92,7 @@ class _BottomAppBarDemoState extends State<BottomAppBarDemo> {
       "When the Scaffold's floating action button location changes, "
       'the floating action button animates to its new position. '
       'The BottomAppBar adapts its shape appropriately.';
-    _scaffoldMessengerKey.currentState.showSnackBar(
+    _scaffoldMessengerKey.currentState!.showSnackBar(
       const SnackBar(content: Text(text)),
     );
   }
@@ -111,27 +111,27 @@ class _BottomAppBarDemoState extends State<BottomAppBarDemo> {
   _ChoiceValue<Widget> _fabShape = kCircularFab;
   _ChoiceValue<bool> _showNotch = kShowNotchTrue;
   _ChoiceValue<FloatingActionButtonLocation> _fabLocation = kFabEndDocked;
-  Color _babColor = kBabColors.first.color;
+  Color? _babColor = kBabColors.first.color;
 
-  void _onShowNotchChanged(_ChoiceValue<bool> value) {
+  void _onShowNotchChanged(_ChoiceValue<bool>? value) {
     setState(() {
-      _showNotch = value;
+      _showNotch = value!;
     });
   }
 
-  void _onFabShapeChanged(_ChoiceValue<Widget> value) {
+  void _onFabShapeChanged(_ChoiceValue<Widget>? value) {
     setState(() {
-      _fabShape = value;
+      _fabShape = value!;
     });
   }
 
-  void _onFabLocationChanged(_ChoiceValue<FloatingActionButtonLocation> value) {
+  void _onFabLocationChanged(_ChoiceValue<FloatingActionButtonLocation>? value) {
     setState(() {
-      _fabLocation = value;
+      _fabLocation = value!;
     });
   }
 
-  void _onBabColorChanged(Color value) {
+  void _onBabColorChanged(Color? value) {
     setState(() {
       _babColor = value;
     });
@@ -201,8 +201,8 @@ class _BottomAppBarDemoState extends State<BottomAppBarDemo> {
     );
   }
 
-  NotchedShape _selectNotch() {
-    if (!_showNotch.value)
+  NotchedShape? _selectNotch() {
+    if (!_showNotch.value!)
       return null;
     if (_fabShape == kCircularFab)
       return const CircularNotchedRectangle();
@@ -215,9 +215,9 @@ class _BottomAppBarDemoState extends State<BottomAppBarDemo> {
 class _ChoiceValue<T> {
   const _ChoiceValue({ this.value, this.title, this.label });
 
-  final T value;
-  final String title;
-  final String label; // For the Semantics widget that contains title
+  final T? value;
+  final String? title;
+  final String? label; // For the Semantics widget that contains title
 
   @override
   String toString() => '$runtimeType("$title")';
@@ -228,7 +228,7 @@ class _RadioItem<T> extends StatelessWidget {
 
   final _ChoiceValue<T> value;
   final _ChoiceValue<T> groupValue;
-  final ValueChanged<_ChoiceValue<T>> onChanged;
+  final ValueChanged<_ChoiceValue<T>?> onChanged;
 
   @override
   Widget build(BuildContext context) {
@@ -256,7 +256,7 @@ class _RadioItem<T> extends StatelessWidget {
                     onChanged(value);
                   },
                   child: Text(
-                    value.title,
+                    value.title!,
                     style: theme.textTheme.subtitle1,
                   ),
                 ),
@@ -272,7 +272,7 @@ class _RadioItem<T> extends StatelessWidget {
 class _NamedColor {
   const _NamedColor(this.color, this.name);
 
-  final Color color;
+  final Color? color;
   final String name;
 }
 
@@ -280,8 +280,8 @@ class _ColorsItem extends StatelessWidget {
   const _ColorsItem(this.colors, this.selectedColor, this.onChanged);
 
   final List<_NamedColor> colors;
-  final Color selectedColor;
-  final ValueChanged<Color> onChanged;
+  final Color? selectedColor;
+  final ValueChanged<Color?> onChanged;
 
   @override
   Widget build(BuildContext context) {
@@ -340,9 +340,9 @@ class _DemoBottomAppBar extends StatelessWidget {
     this.shape,
   });
 
-  final Color color;
-  final FloatingActionButtonLocation fabLocation;
-  final NotchedShape shape;
+  final Color? color;
+  final FloatingActionButtonLocation? fabLocation;
+  final NotchedShape? shape;
 
   static final List<FloatingActionButtonLocation> kCenterLocations = <FloatingActionButtonLocation>[
     FloatingActionButtonLocation.centerDocked,
@@ -421,8 +421,8 @@ class _DiamondFab extends StatelessWidget {
     this.onPressed,
   });
 
-  final Widget child;
-  final VoidCallback onPressed;
+  final Widget? child;
+  final VoidCallback? onPressed;
 
   @override
   Widget build(BuildContext context) {
@@ -436,7 +436,7 @@ class _DiamondFab extends StatelessWidget {
           height: 56.0,
           child: IconTheme.merge(
             data: IconThemeData(color: Theme.of(context).accentIconTheme.color),
-            child: child,
+            child: child!,
           ),
         ),
       ),
@@ -449,8 +449,8 @@ class _DiamondNotchedRectangle implements NotchedShape {
   const _DiamondNotchedRectangle();
 
   @override
-  Path getOuterPath(Rect host, Rect guest) {
-    if (!host.overlaps(guest))
+  Path getOuterPath(Rect host, Rect? guest) {
+    if (!host.overlaps(guest!))
       return Path()..addRect(host);
     assert(guest.width > 0.0);
 
@@ -492,12 +492,12 @@ class _DiamondBorder extends ShapeBorder {
   }
 
   @override
-  Path getInnerPath(Rect rect, { TextDirection textDirection }) {
+  Path getInnerPath(Rect rect, { TextDirection? textDirection }) {
     return getOuterPath(rect, textDirection: textDirection);
   }
 
   @override
-  Path getOuterPath(Rect rect, { TextDirection textDirection }) {
+  Path getOuterPath(Rect rect, { TextDirection? textDirection }) {
     return Path()
       ..moveTo(rect.left + rect.width / 2.0, rect.top)
       ..lineTo(rect.right, rect.top + rect.height / 2.0)
@@ -507,11 +507,11 @@ class _DiamondBorder extends ShapeBorder {
   }
 
   @override
-  void paint(Canvas canvas, Rect rect, { TextDirection textDirection }) { }
+  void paint(Canvas canvas, Rect rect, { TextDirection? textDirection }) { }
 
   // This border doesn't support scaling.
   @override
   ShapeBorder scale(double t) {
-    return null;
+    return this;
   }
 }

dev/integration_tests/flutter_gallery/lib/demo/material/bottom_navigation_demo.dart

@@ -8,11 +8,11 @@ import '../../gallery/demo.dart';
 
 class NavigationIconView {
   NavigationIconView({
-    Widget icon,
-    Widget activeIcon,
-    String title,
-    Color color,
-    TickerProvider vsync,
+    required Widget icon,
+    Widget? activeIcon,
+    String? title,
+    Color? color,
+    required TickerProvider vsync,
   }) : _icon = icon,
        _color = color,
        _title = title,
@@ -32,14 +32,14 @@ class NavigationIconView {
   }
 
   final Widget _icon;
-  final Color _color;
-  final String _title;
+  final Color? _color;
+  final String? _title;
   final BottomNavigationBarItem item;
   final AnimationController controller;
-  Animation<double> _animation;
+  late Animation<double> _animation;
 
   FadeTransition transition(BottomNavigationBarType type, BuildContext context) {
-    Color iconColor;
+    Color? iconColor;
     if (type == BottomNavigationBarType.shifting) {
       iconColor = _color;
     } else {
@@ -79,8 +79,8 @@ class CustomIcon extends StatelessWidget {
     final IconThemeData iconTheme = IconTheme.of(context);
     return Container(
       margin: const EdgeInsets.all(4.0),
-      width: iconTheme.size - 8.0,
-      height: iconTheme.size - 8.0,
+      width: iconTheme.size! - 8.0,
+      height: iconTheme.size! - 8.0,
       color: iconTheme.color,
     );
   }
@@ -92,10 +92,10 @@ class CustomInactiveIcon extends StatelessWidget {
     final IconThemeData iconTheme = IconTheme.of(context);
     return Container(
       margin: const EdgeInsets.all(4.0),
-      width: iconTheme.size - 8.0,
-      height: iconTheme.size - 8.0,
+      width: iconTheme.size! - 8.0,
+      height: iconTheme.size! - 8.0,
       decoration: BoxDecoration(
-        border: Border.all(color: iconTheme.color, width: 2.0),
+        border: Border.all(color: iconTheme.color!, width: 2.0),
       ),
     );
   }
@@ -112,7 +112,7 @@ class _BottomNavigationDemoState extends State<BottomNavigationDemo>
     with TickerProviderStateMixin {
   int _currentIndex = 0;
   BottomNavigationBarType _type = BottomNavigationBarType.shifting;
-  List<NavigationIconView> _navigationViews;
+  late List<NavigationIconView> _navigationViews;
 
   @override
   void initState() {

dev/integration_tests/flutter_gallery/lib/demo/material/buttons_demo.dart

@@ -53,7 +53,7 @@ class ButtonsDemo extends StatefulWidget {
 }
 
 class _ButtonsDemoState extends State<ButtonsDemo> {
-  OutlinedBorder _buttonShape;
+  OutlinedBorder? _buttonShape;
 
   @override
   Widget build(BuildContext context) {
@@ -118,7 +118,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
     );
   }
 
-  Widget buildElevatedButton(OutlinedBorder shape) {
+  Widget buildElevatedButton(OutlinedBorder? shape) {
     final ButtonStyle style = ElevatedButton.styleFrom(shape: shape);
     return Align(
       alignment: const Alignment(0.0, -0.2),
@@ -156,7 +156,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
                 style: style,
                 icon: const Icon(Icons.add, size: 18.0),
                 label: const Text('DISABLED', semanticsLabel: 'DISABLED BUTTON 2'),
-                onPressed: null,
+                onPressed: () {},
               ),
             ],
           ),
@@ -165,7 +165,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
     );
   }
 
-  Widget buildTextButton(OutlinedBorder shape) {
+  Widget buildTextButton(OutlinedBorder? shape) {
     final ButtonStyle style = ElevatedButton.styleFrom(shape: shape);
     return Align(
       alignment: const Alignment(0.0, -0.2),
@@ -203,7 +203,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
                 style: style,
                 icon: const Icon(Icons.add_circle_outline, size: 18.0),
                 label: const Text('DISABLED', semanticsLabel: 'DISABLED BUTTON 4'),
-                onPressed: null,
+                onPressed: () {},
               ),
             ],
           ),
@@ -212,7 +212,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
     );
   }
 
-  Widget buildOutlinedButton(OutlinedBorder shape) {
+  Widget buildOutlinedButton(OutlinedBorder? shape) {
     final ButtonStyle style = ElevatedButton.styleFrom(shape: shape);
     return Align(
       alignment: const Alignment(0.0, -0.2),
@@ -260,9 +260,9 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
   }
 
   // https://en.wikipedia.org/wiki/Free_Four
-  String dropdown1Value = 'Free';
-  String dropdown2Value;
-  String dropdown3Value = 'Four';
+  String? dropdown1Value = 'Free';
+  String? dropdown2Value;
+  String? dropdown3Value = 'Four';
 
   Widget buildDropdownButton() {
     return Padding(
@@ -274,7 +274,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
             title: const Text('Simple dropdown:'),
             trailing: DropdownButton<String>(
               value: dropdown1Value,
-              onChanged: (String newValue) {
+              onChanged: (String? newValue) {
                 setState(() {
                   dropdown1Value = newValue;
                 });
@@ -295,7 +295,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
             trailing: DropdownButton<String>(
               value: dropdown2Value,
               hint: const Text('Choose'),
-              onChanged: (String newValue) {
+              onChanged: (String? newValue) {
                 setState(() {
                   dropdown2Value = newValue;
                 });
@@ -315,7 +315,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
             title: const Text('Scrollable dropdown:'),
             trailing: DropdownButton<String>(
               value: dropdown3Value,
-              onChanged: (String newValue) {
+              onChanged: (String? newValue) {
                 setState(() {
                   dropdown3Value = newValue;
                 });

dev/integration_tests/flutter_gallery/lib/demo/material/cards_demo.dart

@@ -17,19 +17,14 @@ enum CardDemoType {
 
 class TravelDestination {
   const TravelDestination({
-    @required this.assetName,
-    @required this.assetPackage,
-    @required this.title,
-    @required this.description,
-    @required this.city,
-    @required this.location,
+    required this.assetName,
+    required this.assetPackage,
+    required this.title,
+    required this.description,
+    required this.city,
+    required this.location,
     this.type = CardDemoType.standard,
-  }) : assert(assetName != null),
-       assert(assetPackage != null),
-       assert(title != null),
-       assert(description != null),
-       assert(city != null),
-       assert(location != null);
+  });
 
   final String assetName;
   final String assetPackage;
@@ -70,14 +65,13 @@ const List<TravelDestination> destinations = <TravelDestination>[
 ];
 
 class TravelDestinationItem extends StatelessWidget {
-  const TravelDestinationItem({ Key key, @required this.destination, this.shape })
-    : assert(destination != null),
-      super(key: key);
+  const TravelDestinationItem({ Key? key, required this.destination, this.shape })
+    : super(key: key);
 
   // This height will allow for all the Card's content to fit comfortably within the card.
   static const double height = 338.0;
   final TravelDestination destination;
-  final ShapeBorder shape;
+  final ShapeBorder? shape;
 
   @override
   Widget build(BuildContext context) {
@@ -106,14 +100,13 @@ class TravelDestinationItem extends StatelessWidget {
 }
 
 class TappableTravelDestinationItem extends StatelessWidget {
-  const TappableTravelDestinationItem({ Key key, @required this.destination, this.shape })
-    : assert(destination != null),
-      super(key: key);
+  const TappableTravelDestinationItem({ Key? key, required this.destination, this.shape })
+    : super(key: key);
 
   // This height will allow for all the Card's content to fit comfortably within the card.
   static const double height = 298.0;
   final TravelDestination destination;
-  final ShapeBorder shape;
+  final ShapeBorder? shape;
 
   @override
   Widget build(BuildContext context) {
@@ -151,12 +144,11 @@ class TappableTravelDestinationItem extends StatelessWidget {
 }
 
 class SelectableTravelDestinationItem extends StatefulWidget {
-  const SelectableTravelDestinationItem({ Key key, @required this.destination, this.shape })
-    : assert(destination != null),
-      super(key: key);
+  const SelectableTravelDestinationItem({ Key? key, required this.destination, this.shape })
+    : super(key: key);
 
   final TravelDestination destination;
-  final ShapeBorder shape;
+  final ShapeBorder? shape;
 
   @override
   _SelectableTravelDestinationItemState createState() => _SelectableTravelDestinationItemState();
@@ -231,11 +223,11 @@ class _SelectableTravelDestinationItemState extends State<SelectableTravelDestin
 
 class SectionTitle extends StatelessWidget {
   const SectionTitle({
-    Key key,
+    Key? key,
     this.title,
   }) : super(key: key);
 
-  final String title;
+  final String? title;
 
   @override
   Widget build(BuildContext context) {
@@ -243,24 +235,23 @@ class SectionTitle extends StatelessWidget {
       padding: const EdgeInsets.fromLTRB(4.0, 4.0, 4.0, 12.0),
       child: Align(
         alignment: Alignment.centerLeft,
-        child: Text(title, style: Theme.of(context).textTheme.subtitle1),
+        child: Text(title!, style: Theme.of(context).textTheme.subtitle1),
       ),
     );
   }
 }
 
 class TravelDestinationContent extends StatelessWidget {
-  const TravelDestinationContent({ Key key, @required this.destination })
-    : assert(destination != null),
-      super(key: key);
+  const TravelDestinationContent({ Key? key, required this.destination })
+    : super(key: key);
 
   final TravelDestination destination;
 
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
-    final TextStyle titleStyle = theme.textTheme.headline5.copyWith(color: Colors.white);
-    final TextStyle descriptionStyle = theme.textTheme.subtitle1;
+    final TextStyle titleStyle = theme.textTheme.headline5!.copyWith(color: Colors.white);
+    final TextStyle descriptionStyle = theme.textTheme.subtitle1!;
     final ButtonStyle textButtonStyle = TextButton.styleFrom(primary: Colors.amber.shade500);
 
     return Column(
@@ -352,7 +343,7 @@ class CardsDemo extends StatefulWidget {
 }
 
 class _CardsDemoState extends State<CardsDemo> {
-  ShapeBorder _shape;
+  ShapeBorder? _shape;
 
   @override
   Widget build(BuildContext context) {
@@ -385,7 +376,7 @@ class _CardsDemoState extends State<CardsDemo> {
         child: ListView(
           padding: const EdgeInsets.only(top: 8.0, left: 8.0, right: 8.0),
           children: destinations.map<Widget>((TravelDestination destination) {
-            Widget child;
+            Widget? child;
             switch (destination.type) {
               case CardDemoType.standard:
                 child = TravelDestinationItem(destination: destination, shape: _shape);

dev/integration_tests/flutter_gallery/lib/demo/material/chip_demo.dart

@@ -88,13 +88,13 @@ const Map<String, Set<String>> _materialActions = <String, Set<String>>{
 
 class _ChipsTile extends StatelessWidget {
   const _ChipsTile({
-    Key key,
+    Key? key,
     this.label,
     this.children,
   }) : super(key: key);
 
-  final String label;
-  final List<Widget> children;
+  final String? label;
+  final List<Widget>? children;
 
   // Wraps a list of chips into a ListTile for display as a section in the demo.
   @override
@@ -107,11 +107,11 @@ class _ChipsTile extends StatelessWidget {
           Container(
             padding: const EdgeInsets.only(top: 16.0, bottom: 4.0),
             alignment: Alignment.center,
-            child: Text(label, textAlign: TextAlign.start),
+            child: Text(label!, textAlign: TextAlign.start),
           ),
-          if (children.isNotEmpty)
+          if (children!.isNotEmpty)
             Wrap(
-              children: children.map<Widget>((Widget chip) {
+              children: children!.map<Widget>((Widget chip) {
                 return Padding(
                   padding: const EdgeInsets.all(2.0),
                   child: chip,
@@ -125,7 +125,7 @@ class _ChipsTile extends StatelessWidget {
                 alignment: Alignment.center,
                 constraints: const BoxConstraints(minWidth: 48.0, minHeight: 48.0),
                 padding: const EdgeInsets.all(8.0),
-                child: Text('None', style: Theme.of(context).textTheme.caption.copyWith(fontStyle: FontStyle.italic)),
+                child: Text('None', style: Theme.of(context).textTheme.caption!.copyWith(fontStyle: FontStyle.italic)),
               ),
             ),
         ],
@@ -188,7 +188,7 @@ class _ChipDemoState extends State<ChipDemo> {
   }
 
   String _capitalize(String name) {
-    assert(name != null && name.isNotEmpty);
+    assert(name.isNotEmpty);
     return name.substring(0, 1).toUpperCase() + name.substring(1);
   }
 
@@ -209,7 +209,7 @@ class _ChipDemoState extends State<ChipDemo> {
   AssetImage _nameToAvatar(String name) {
     assert(_avatars.containsKey(name));
     return AssetImage(
-      _avatars[name],
+      _avatars[name]!,
       package: 'flutter_gallery_assets',
     );
   }
@@ -218,7 +218,7 @@ class _ChipDemoState extends State<ChipDemo> {
     if (_selectedAction.isEmpty) {
       return '';
     }
-    return _capitalize(_results[_selectedAction]) + '!';
+    return _capitalize(_results[_selectedAction]!) + '!';
   }
 
   @override
@@ -285,11 +285,11 @@ class _ChipDemoState extends State<ChipDemo> {
     }).toList();
 
     Set<String> allowedActions = <String>{};
-    if (_selectedMaterial != null && _selectedMaterial.isNotEmpty) {
+    if (_selectedMaterial.isNotEmpty) {
       for (final String tool in _selectedTools) {
-        allowedActions.addAll(_toolActions[tool]);
+        allowedActions.addAll(_toolActions[tool]!);
       }
-      allowedActions = allowedActions.intersection(_materialActions[_selectedMaterial]);
+      allowedActions = allowedActions.intersection(_materialActions[_selectedMaterial]!);
     }
 
     final List<Widget> actionChips = allowedActions.map<Widget>((String name) {

dev/integration_tests/flutter_gallery/lib/demo/material/data_table_demo.dart

@@ -18,7 +18,7 @@ class Dessert {
   final int calcium;
   final int iron;
 
-  bool selected = false;
+  bool? selected = false;
 }
 
 class DessertDataSource extends DataTableSource {
@@ -96,17 +96,17 @@ class DessertDataSource extends DataTableSource {
   int _selectedCount = 0;
 
   @override
-  DataRow getRow(int index) {
+  DataRow? getRow(int index) {
     assert(index >= 0);
     if (index >= _desserts.length)
       return null;
     final Dessert dessert = _desserts[index];
     return DataRow.byIndex(
       index: index,
-      selected: dessert.selected,
-      onSelectChanged: (bool value) {
+      selected: dessert.selected!,
+      onSelectChanged: (bool? value) {
         if (dessert.selected != value) {
-          _selectedCount += value ? 1 : -1;
+          _selectedCount += value! ? 1 : -1;
           assert(_selectedCount >= 0);
           dessert.selected = value;
           notifyListeners();
@@ -134,10 +134,10 @@ class DessertDataSource extends DataTableSource {
   @override
   int get selectedRowCount => _selectedCount;
 
-  void _selectAll(bool checked) {
+  void _selectAll(bool? checked) {
     for (final Dessert dessert in _desserts)
       dessert.selected = checked;
-    _selectedCount = checked ? _desserts.length : 0;
+    _selectedCount = checked! ? _desserts.length : 0;
     notifyListeners();
   }
 }
@@ -150,8 +150,8 @@ class DataTableDemo extends StatefulWidget {
 }
 
 class _DataTableDemoState extends State<DataTableDemo> {
-  int _rowsPerPage = PaginatedDataTable.defaultRowsPerPage;
-  int _sortColumnIndex;
+  int? _rowsPerPage = PaginatedDataTable.defaultRowsPerPage;
+  int? _sortColumnIndex;
   bool _sortAscending = true;
   final DessertDataSource _dessertsDataSource = DessertDataSource();
 
@@ -178,8 +178,8 @@ class _DataTableDemoState extends State<DataTableDemo> {
           children: <Widget>[
             PaginatedDataTable(
               header: const Text('Nutrition'),
-              rowsPerPage: _rowsPerPage,
-              onRowsPerPageChanged: (int value) { setState(() { _rowsPerPage = value; }); },
+              rowsPerPage: _rowsPerPage!,
+              onRowsPerPageChanged: (int? value) { setState(() { _rowsPerPage = value; }); },
               sortColumnIndex: _sortColumnIndex,
               sortAscending: _sortAscending,
               onSelectAll: _dessertsDataSource._selectAll,

dev/integration_tests/flutter_gallery/lib/demo/material/date_and_time_picker_demo.dart

@@ -9,7 +9,7 @@ import '../../gallery/demo.dart';
 
 class _InputDropdown extends StatelessWidget {
   const _InputDropdown({
-    Key key,
+    Key? key,
     this.child,
     this.labelText,
     this.valueText,
@@ -17,11 +17,11 @@ class _InputDropdown extends StatelessWidget {
     this.onPressed,
   }) : super(key: key);
 
-  final String labelText;
-  final String valueText;
-  final TextStyle valueStyle;
-  final VoidCallback onPressed;
-  final Widget child;
+  final String? labelText;
+  final String? valueText;
+  final TextStyle? valueStyle;
+  final VoidCallback? onPressed;
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {
@@ -36,7 +36,7 @@ class _InputDropdown extends StatelessWidget {
           mainAxisAlignment: MainAxisAlignment.spaceBetween,
           mainAxisSize: MainAxisSize.min,
           children: <Widget>[
-            Text(valueText, style: valueStyle),
+            Text(valueText!, style: valueStyle),
             Icon(Icons.arrow_drop_down,
               color: Theme.of(context).brightness == Brightness.light ? Colors.grey.shade700 : Colors.white70,
             ),
@@ -49,7 +49,7 @@ class _InputDropdown extends StatelessWidget {
 
 class _DateTimePicker extends StatelessWidget {
   const _DateTimePicker({
-    Key key,
+    Key? key,
     this.labelText,
     this.selectedDate,
     this.selectedTime,
@@ -57,35 +57,35 @@ class _DateTimePicker extends StatelessWidget {
     this.selectTime,
   }) : super(key: key);
 
-  final String labelText;
-  final DateTime selectedDate;
-  final TimeOfDay selectedTime;
-  final ValueChanged<DateTime> selectDate;
-  final ValueChanged<TimeOfDay> selectTime;
+  final String? labelText;
+  final DateTime? selectedDate;
+  final TimeOfDay? selectedTime;
+  final ValueChanged<DateTime>? selectDate;
+  final ValueChanged<TimeOfDay>? selectTime;
 
   Future<void> _selectDate(BuildContext context) async {
-    final DateTime picked = await showDatePicker(
+    final DateTime? picked = await showDatePicker(
       context: context,
-      initialDate: selectedDate,
+      initialDate: selectedDate!,
       firstDate: DateTime(2015, 8),
       lastDate: DateTime(2101),
     );
     if (picked != null && picked != selectedDate)
-      selectDate(picked);
+      selectDate!(picked);
   }
 
   Future<void> _selectTime(BuildContext context) async {
-    final TimeOfDay picked = await showTimePicker(
+    final TimeOfDay? picked = await showTimePicker(
       context: context,
-      initialTime: selectedTime,
+      initialTime: selectedTime!,
     );
     if (picked != null && picked != selectedTime)
-      selectTime(picked);
+      selectTime!(picked);
   }
 
   @override
   Widget build(BuildContext context) {
-    final TextStyle valueStyle = Theme.of(context).textTheme.headline6;
+    final TextStyle? valueStyle = Theme.of(context).textTheme.headline6;
     return Row(
       crossAxisAlignment: CrossAxisAlignment.end,
       children: <Widget>[
@@ -93,7 +93,7 @@ class _DateTimePicker extends StatelessWidget {
           flex: 4,
           child: _InputDropdown(
             labelText: labelText,
-            valueText: DateFormat.yMMMd().format(selectedDate),
+            valueText: DateFormat.yMMMd().format(selectedDate!),
             valueStyle: valueStyle,
             onPressed: () { _selectDate(context); },
           ),
@@ -102,7 +102,7 @@ class _DateTimePicker extends StatelessWidget {
         Expanded(
           flex: 3,
           child: _InputDropdown(
-            valueText: selectedTime.format(context),
+            valueText: selectedTime!.format(context),
             valueStyle: valueStyle,
             onPressed: () { _selectTime(context); },
           ),
@@ -125,7 +125,7 @@ class _DateAndTimePickerDemoState extends State<DateAndTimePickerDemo> {
   DateTime _toDate = DateTime.now();
   TimeOfDay _toTime = const TimeOfDay(hour: 8, minute: 28);
   final List<String> _allActivities = <String>['hiking', 'swimming', 'boating', 'fishing'];
-  String _activity = 'fishing';
+  String? _activity = 'fishing';
 
   @override
   Widget build(BuildContext context) {
@@ -153,7 +153,7 @@ class _DateAndTimePickerDemoState extends State<DateAndTimePickerDemo> {
                 decoration: const InputDecoration(
                   labelText: 'Location',
                 ),
-                style: Theme.of(context).textTheme.headline4.copyWith(fontSize: 20.0),
+                style: Theme.of(context).textTheme.headline4!.copyWith(fontSize: 20.0),
               ),
               _DateTimePicker(
                 labelText: 'From',
@@ -195,7 +195,7 @@ class _DateAndTimePickerDemoState extends State<DateAndTimePickerDemo> {
                 isEmpty: _activity == null,
                 child: DropdownButton<String>(
                   value: _activity,
-                  onChanged: (String newValue) {
+                  onChanged: (String? newValue) {
                     setState(() {
                       _activity = newValue;
                     });

dev/integration_tests/flutter_gallery/lib/demo/material/dialog_demo.dart

@@ -21,12 +21,12 @@ const String _alertWithTitleText =
   'data to Google, even when no apps are running.';
 
 class DialogDemoItem extends StatelessWidget {
-  const DialogDemoItem({ Key key, this.icon, this.color, this.text, this.onPressed }) : super(key: key);
+  const DialogDemoItem({ Key? key, this.icon, this.color, this.text, this.onPressed }) : super(key: key);
 
-  final IconData icon;
-  final Color color;
-  final String text;
-  final VoidCallback onPressed;
+  final IconData? icon;
+  final Color? color;
+  final String? text;
+  final VoidCallback? onPressed;
 
   @override
   Widget build(BuildContext context) {
@@ -39,7 +39,7 @@ class DialogDemoItem extends StatelessWidget {
           Icon(icon, size: 36.0, color: color),
           Padding(
             padding: const EdgeInsets.only(left: 16.0),
-            child: Text(text),
+            child: Text(text!),
           ),
         ],
       ),
@@ -56,7 +56,7 @@ class DialogDemo extends StatefulWidget {
 
 class DialogDemoState extends State<DialogDemo> {
 
-  TimeOfDay _selectedTime;
+  TimeOfDay? _selectedTime;
 
   @override
   void initState() {
@@ -65,12 +65,12 @@ class DialogDemoState extends State<DialogDemo> {
     _selectedTime = TimeOfDay(hour: now.hour, minute: now.minute);
   }
 
-  void showDemoDialog<T>({ BuildContext context, Widget child }) {
+  void showDemoDialog<T>({ required BuildContext context, Widget? child }) {
     showDialog<T>(
       context: context,
-      builder: (BuildContext context) => child,
+      builder: (BuildContext context) => child!,
     )
-    .then<void>((T value) { // The value passed to Navigator.pop() or null.
+    .then((T? value) { // The value passed to Navigator.pop() or null.
       if (value != null) {
         ScaffoldMessenger.of(context).showSnackBar(SnackBar(
           content: Text('You selected: $value'),
@@ -82,7 +82,7 @@ class DialogDemoState extends State<DialogDemo> {
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
-    final TextStyle dialogTextStyle = theme.textTheme.subtitle1.copyWith(color: theme.textTheme.caption.color);
+    final TextStyle dialogTextStyle = theme.textTheme.subtitle1!.copyWith(color: theme.textTheme.caption!.color);
 
     return Scaffold(
       appBar: AppBar(
@@ -176,9 +176,9 @@ class DialogDemoState extends State<DialogDemo> {
             onPressed: () {
               showTimePicker(
                 context: context,
-                initialTime: _selectedTime,
+                initialTime: _selectedTime!,
               )
-              .then<void>((TimeOfDay value) {
+              .then((TimeOfDay? value) {
                 if (value != null && value != _selectedTime) {
                   _selectedTime = value;
                   ScaffoldMessenger.of(context).showSnackBar(SnackBar(

dev/integration_tests/flutter_gallery/lib/demo/material/drawer_demo.dart

@@ -33,9 +33,9 @@ class _DrawerDemoState extends State<DrawerDemo> with TickerProviderStateMixin {
     curve: Curves.fastOutSlowIn,
   ));
 
-  AnimationController _controller;
-  Animation<double> _drawerContentsOpacity;
-  Animation<Offset> _drawerDetailsPosition;
+  late AnimationController _controller;
+  late Animation<double> _drawerContentsOpacity;
+  late Animation<Offset> _drawerDetailsPosition;
   bool _showDrawerContents = true;
 
   @override
@@ -58,7 +58,7 @@ class _DrawerDemoState extends State<DrawerDemo> with TickerProviderStateMixin {
     super.dispose();
   }
 
-  IconData _backIcon() {
+  IconData? _backIcon() {
     switch (Theme.of(context).platform) {
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
@@ -69,8 +69,6 @@ class _DrawerDemoState extends State<DrawerDemo> with TickerProviderStateMixin {
       case TargetPlatform.macOS:
         return Icons.arrow_back_ios;
     }
-    assert(false);
-    return null;
   }
 
   void _showNotImplementedMessage() {
@@ -211,7 +209,7 @@ class _DrawerDemoState extends State<DrawerDemo> with TickerProviderStateMixin {
       body: Center(
         child: InkWell(
           onTap: () {
-            _scaffoldKey.currentState.openDrawer();
+            _scaffoldKey.currentState!.openDrawer();
           },
           child: Semantics(
             button: true,

dev/integration_tests/flutter_gallery/lib/demo/material/expansion_panels_demo.dart

@@ -14,7 +14,7 @@ enum Location {
 }
 
 typedef DemoItemBodyBuilder<T> = Widget Function(DemoItem<T> item);
-typedef ValueToString<T> = String Function(T value);
+typedef ValueToString<T> = String? Function(T value);
 
 class DualHeaderWithHint extends StatelessWidget {
   const DualHeaderWithHint({
@@ -24,10 +24,10 @@ class DualHeaderWithHint extends StatelessWidget {
     this.showHint,
   });
 
-  final String name;
-  final String value;
-  final String hint;
-  final bool showHint;
+  final String? name;
+  final String? value;
+  final String? hint;
+  final bool? showHint;
 
   Widget _crossFade(Widget first, Widget second, bool isExpanded) {
     return AnimatedCrossFade(
@@ -56,8 +56,8 @@ class DualHeaderWithHint extends StatelessWidget {
               fit: BoxFit.scaleDown,
               alignment: Alignment.centerLeft,
               child: Text(
-                name,
-                style: textTheme.bodyText2.copyWith(fontSize: 15.0),
+                name!,
+                style: textTheme.bodyText2!.copyWith(fontSize: 15.0),
               ),
             ),
           ),
@@ -67,9 +67,9 @@ class DualHeaderWithHint extends StatelessWidget {
           child: Container(
             margin: const EdgeInsets.only(left: 24.0),
             child: _crossFade(
-              Text(value, style: textTheme.caption.copyWith(fontSize: 15.0)),
-              Text(hint, style: textTheme.caption.copyWith(fontSize: 15.0)),
-              showHint,
+              Text(value!, style: textTheme.caption!.copyWith(fontSize: 15.0)),
+              Text(hint!, style: textTheme.caption!.copyWith(fontSize: 15.0)),
+              showHint!,
             ),
           ),
         ),
@@ -87,9 +87,9 @@ class CollapsibleBody extends StatelessWidget {
   });
 
   final EdgeInsets margin;
-  final Widget child;
-  final VoidCallback onSave;
-  final VoidCallback onCancel;
+  final Widget? child;
+  final VoidCallback? onSave;
+  final VoidCallback? onCancel;
 
   @override
   Widget build(BuildContext context) {
@@ -106,8 +106,8 @@ class CollapsibleBody extends StatelessWidget {
           ) - margin,
           child: Center(
             child: DefaultTextStyle(
-              style: textTheme.caption.copyWith(fontSize: 15.0),
-              child: child,
+              style: textTheme.caption!.copyWith(fontSize: 15.0),
+              child: child!,
             ),
           ),
         ),
@@ -149,15 +149,15 @@ class DemoItem<T> {
     this.value,
     this.hint,
     this.builder,
-    this.valueToString,
+    required this.valueToString,
   }) : textController = TextEditingController(text: valueToString(value));
 
-  final String name;
-  final String hint;
+  final String? name;
+  final String? hint;
   final TextEditingController textController;
-  final DemoItemBodyBuilder<T> builder;
-  final ValueToString<T> valueToString;
-  T value;
+  final DemoItemBodyBuilder<T>? builder;
+  final ValueToString<T?> valueToString;
+  T? value;
   bool isExpanded = false;
 
   ExpansionPanelHeaderBuilder get headerBuilder {
@@ -171,7 +171,7 @@ class DemoItem<T> {
     };
   }
 
-  Widget build() => builder(this);
+  Widget build() => builder!(this);
 }
 
 class ExpansionPanelsDemo extends StatefulWidget {
@@ -182,7 +182,7 @@ class ExpansionPanelsDemo extends StatefulWidget {
 }
 
 class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
-  List<DemoItem<dynamic>> _demoItems;
+  late List<DemoItem<dynamic>> _demoItems;
 
   @override
   void initState() {
@@ -193,7 +193,7 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
         name: 'Trip',
         value: 'Caribbean cruise',
         hint: 'Change trip name',
-        valueToString: (String value) => value,
+        valueToString: (String? value) => value,
         builder: (DemoItem<String> item) {
           void close() {
             setState(() {
@@ -206,8 +206,8 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
               builder: (BuildContext context) {
                 return CollapsibleBody(
                   margin: const EdgeInsets.symmetric(horizontal: 16.0),
-                  onSave: () { Form.of(context).save(); close(); },
-                  onCancel: () { Form.of(context).reset(); close(); },
+                  onSave: () { Form.of(context)!.save(); close(); },
+                  onCancel: () { Form.of(context)!.reset(); close(); },
                   child: Padding(
                     padding: const EdgeInsets.symmetric(horizontal: 16.0),
                     child: TextFormField(
@@ -216,7 +216,7 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
                         hintText: item.hint,
                         labelText: item.name,
                       ),
-                      onSaved: (String value) { item.value = value; },
+                      onSaved: (String? value) { item.value = value; },
                     ),
                   ),
                 );
@@ -229,7 +229,7 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
         name: 'Location',
         value: Location.Bahamas,
         hint: 'Select location',
-        valueToString: (Location location) => location.toString().split('.')[1],
+        valueToString: (Location? location) => location.toString().split('.')[1],
         builder: (DemoItem<Location> item) {
           void close() {
             setState(() {
@@ -240,11 +240,11 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
             child: Builder(
               builder: (BuildContext context) {
                 return CollapsibleBody(
-                  onSave: () { Form.of(context).save(); close(); },
-                  onCancel: () { Form.of(context).reset(); close(); },
+                  onSave: () { Form.of(context)!.save(); close(); },
+                  onCancel: () { Form.of(context)!.reset(); close(); },
                   child: FormField<Location>(
                     initialValue: item.value,
-                    onSaved: (Location result) { item.value = result; },
+                    onSaved: (Location? result) { item.value = result; },
                     builder: (FormFieldState<Location> field) {
                       return Column(
                         mainAxisSize: MainAxisSize.min,
@@ -282,7 +282,7 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
         name: 'Sun',
         value: 80.0,
         hint: 'Select sun level',
-        valueToString: (double amount) => '${amount.round()}',
+        valueToString: (double? amount) => '${amount!.round()}',
         builder: (DemoItem<double> item) {
           void close() {
             setState(() {
@@ -294,11 +294,11 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
             child: Builder(
               builder: (BuildContext context) {
                 return CollapsibleBody(
-                  onSave: () { Form.of(context).save(); close(); },
-                  onCancel: () { Form.of(context).reset(); close(); },
+                  onSave: () { Form.of(context)!.save(); close(); },
+                  onCancel: () { Form.of(context)!.reset(); close(); },
                   child: FormField<double>(
                     initialValue: item.value,
-                    onSaved: (double value) { item.value = value; },
+                    onSaved: (double? value) { item.value = value; },
                     builder: (FormFieldState<double> field) {
                       return Container(
                         // Allow room for the value indicator.
@@ -307,9 +307,9 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
                           min: 0.0,
                           max: 100.0,
                           divisions: 5,
-                          activeColor: Colors.orange[100 + (field.value * 5.0).round()],
-                          label: '${field.value.round()}',
-                          value: field.value,
+                          activeColor: Colors.orange[100 + (field.value! * 5.0).round()],
+                          label: '${field.value!.round()}',
+                          value: field.value!,
                           onChanged: field.didChange,
                         ),
                       );

dev/integration_tests/flutter_gallery/lib/demo/material/full_screen_dialog_demo.dart

@@ -15,9 +15,8 @@ enum DismissDialogAction {
 }
 
 class DateTimeItem extends StatelessWidget {
-  DateTimeItem({ Key key, DateTime dateTime, @required this.onChanged })
-    : assert(onChanged != null),
-      date = DateTime(dateTime.year, dateTime.month, dateTime.day),
+  DateTimeItem({ Key? key, required DateTime dateTime, required this.onChanged })
+    : date = DateTime(dateTime.year, dateTime.month, dateTime.day),
       time = TimeOfDay(hour: dateTime.hour, minute: dateTime.minute),
       super(key: key);
 
@@ -30,7 +29,7 @@ class DateTimeItem extends StatelessWidget {
     final ThemeData theme = Theme.of(context);
 
     return DefaultTextStyle(
-      style: theme.textTheme.subtitle1,
+      style: theme.textTheme.subtitle1!,
       child: Row(
         children: <Widget>[
           Expanded(
@@ -47,7 +46,7 @@ class DateTimeItem extends StatelessWidget {
                     firstDate: date.subtract(const Duration(days: 30)),
                     lastDate: date.add(const Duration(days: 30)),
                   )
-                  .then<void>((DateTime value) {
+                  .then((DateTime? value) {
                     if (value != null)
                       onChanged(DateTime(value.year, value.month, value.day, time.hour, time.minute));
                   });
@@ -74,7 +73,7 @@ class DateTimeItem extends StatelessWidget {
                   context: context,
                   initialTime: time,
                 )
-                .then<void>((TimeOfDay value) {
+                .then((TimeOfDay? value) {
                   if (value != null)
                     onChanged(DateTime(date.year, date.month, date.day, value.hour, value.minute));
                 });
@@ -101,11 +100,11 @@ class FullScreenDialogDemo extends StatefulWidget {
 class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
   DateTime _fromDateTime = DateTime.now();
   DateTime _toDateTime = DateTime.now();
-  bool _allDayValue = false;
+  bool? _allDayValue = false;
   bool _saveNeeded = false;
   bool _hasLocation = false;
   bool _hasName = false;
-  String _eventName;
+  late String _eventName;
 
   Future<bool> _onWillPop() async {
     _saveNeeded = _hasLocation || _hasName || _saveNeeded;
@@ -113,9 +112,9 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
       return true;
 
     final ThemeData theme = Theme.of(context);
-    final TextStyle dialogTextStyle = theme.textTheme.subtitle1.copyWith(color: theme.textTheme.caption.color);
+    final TextStyle dialogTextStyle = theme.textTheme.subtitle1!.copyWith(color: theme.textTheme.caption!.color);
 
-    return await showDialog<bool>(
+    return await (showDialog<bool>(
       context: context,
       builder: (BuildContext context) {
         return AlertDialog(
@@ -127,19 +126,19 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
             TextButton(
               child: const Text('CANCEL'),
               onPressed: () {
-                Navigator.of(context).pop(false); // Pops the confirmation dialog but not the page.
+                Navigator.of(context)!.pop(false); // Pops the confirmation dialog but not the page.
               },
             ),
             TextButton(
               child: const Text('DISCARD'),
               onPressed: () {
-                Navigator.of(context).pop(true); // Returning true to _onWillPop will pop again.
+                Navigator.of(context)!.pop(true); // Returning true to _onWillPop will pop again.
               },
             ),
           ],
         );
       },
-    ) ?? false;
+    ) as Future<bool>);
   }
 
   @override
@@ -151,7 +150,7 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
         title: Text(_hasName ? _eventName : 'Event Name TBD'),
         actions: <Widget> [
           TextButton(
-            child: Text('SAVE', style: theme.textTheme.bodyText2.copyWith(color: Colors.white)),
+            child: Text('SAVE', style: theme.textTheme.bodyText2!.copyWith(color: Colors.white)),
             onPressed: () {
               Navigator.pop(context, DismissDialogAction.save);
             },
@@ -238,7 +237,7 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
                   children: <Widget> [
                     Checkbox(
                       value: _allDayValue,
-                      onChanged: (bool value) {
+                      onChanged: (bool? value) {
                         setState(() {
                           _allDayValue = value;
                           _saveNeeded = true;

dev/integration_tests/flutter_gallery/lib/demo/material/grid_list_demo.dart

@@ -26,21 +26,21 @@ class Photo {
     this.isFavorite = false,
   });
 
-  final String assetName;
-  final String assetPackage;
-  final String title;
-  final String caption;
+  final String? assetName;
+  final String? assetPackage;
+  final String? title;
+  final String? caption;
 
   bool isFavorite;
-  String get tag => assetName; // Assuming that all asset names are unique.
+  String? get tag => assetName; // Assuming that all asset names are unique.
 
-  bool get isValid => assetName != null && title != null && caption != null && isFavorite != null;
+  bool get isValid => assetName != null && title != null && caption != null;
 }
 
 class GridPhotoViewer extends StatefulWidget {
-  const GridPhotoViewer({ Key key, this.photo }) : super(key: key);
+  const GridPhotoViewer({ Key? key, this.photo }) : super(key: key);
 
-  final Photo photo;
+  final Photo? photo;
 
   @override
   _GridPhotoViewerState createState() => _GridPhotoViewerState();
@@ -49,25 +49,25 @@ class GridPhotoViewer extends StatefulWidget {
 class _GridTitleText extends StatelessWidget {
   const _GridTitleText(this.text);
 
-  final String text;
+  final String? text;
 
   @override
   Widget build(BuildContext context) {
     return FittedBox(
       fit: BoxFit.scaleDown,
       alignment: Alignment.centerLeft,
-      child: Text(text),
+      child: Text(text!),
     );
   }
 }
 
 class _GridPhotoViewerState extends State<GridPhotoViewer> with SingleTickerProviderStateMixin {
-  AnimationController _controller;
-  Animation<Offset> _flingAnimation;
+  late AnimationController _controller;
+  late Animation<Offset> _flingAnimation;
   Offset _offset = Offset.zero;
   double _scale = 1.0;
-  Offset _normalizedOffset;
-  double _previousScale;
+  late Offset _normalizedOffset;
+  late double _previousScale;
 
   @override
   void initState() {
@@ -85,11 +85,11 @@ class _GridPhotoViewerState extends State<GridPhotoViewer> with SingleTickerProv
   // The maximum offset value is 0,0. If the size of this renderer's box is w,h
   // then the minimum offset value is w - _scale * w, h - _scale * h.
   Offset _clampOffset(Offset offset) {
-    final Size size = context.size;
+    final Size size = context.size!;
     final Offset minOffset = Offset(size.width, size.height) * (1.0 - _scale);
     return Offset(
-      offset.dx.clamp(minOffset.dx, 0.0) as double,
-      offset.dy.clamp(minOffset.dy, 0.0) as double,
+      offset.dx.clamp(minOffset.dx, 0.0),
+      offset.dy.clamp(minOffset.dy, 0.0),
     );
   }
 
@@ -110,7 +110,7 @@ class _GridPhotoViewerState extends State<GridPhotoViewer> with SingleTickerProv
 
   void _handleOnScaleUpdate(ScaleUpdateDetails details) {
     setState(() {
-      _scale = (_previousScale * details.scale).clamp(1.0, 4.0) as double;
+      _scale = (_previousScale * details.scale).clamp(1.0, 4.0);
       // Ensure that image location under the focal point stays in the same place despite scaling.
       _offset = _clampOffset(details.focalPoint - _normalizedOffset * _scale);
     });
@@ -121,7 +121,7 @@ class _GridPhotoViewerState extends State<GridPhotoViewer> with SingleTickerProv
     if (magnitude < _kMinFlingVelocity)
       return;
     final Offset direction = details.velocity.pixelsPerSecond / magnitude;
-    final double distance = (Offset.zero & context.size).shortestSide;
+    final double distance = (Offset.zero & context.size!).shortestSide;
     _flingAnimation = _controller.drive(Tween<Offset>(
       begin: _offset,
       end: _clampOffset(_offset + direction * distance),
@@ -143,8 +143,8 @@ class _GridPhotoViewerState extends State<GridPhotoViewer> with SingleTickerProv
             ..translate(_offset.dx, _offset.dy)
             ..scale(_scale),
           child: Image.asset(
-            widget.photo.assetName,
-            package: widget.photo.assetPackage,
+            widget.photo!.assetName!,
+            package: widget.photo!.assetPackage,
             fit: BoxFit.cover,
           ),
         ),
@@ -155,13 +155,11 @@ class _GridPhotoViewerState extends State<GridPhotoViewer> with SingleTickerProv
 
 class GridDemoPhotoItem extends StatelessWidget {
   GridDemoPhotoItem({
-    Key key,
-    @required this.photo,
-    @required this.tileStyle,
-    @required this.onBannerTap,
-  }) : assert(photo != null && photo.isValid),
-       assert(tileStyle != null),
-       assert(onBannerTap != null),
+    Key? key,
+    required this.photo,
+    required this.tileStyle,
+    required this.onBannerTap,
+  }) : assert(photo.isValid),
        super(key: key);
 
   final Photo photo;
@@ -173,11 +171,11 @@ class GridDemoPhotoItem extends StatelessWidget {
       builder: (BuildContext context) {
         return Scaffold(
           appBar: AppBar(
-            title: Text(photo.title),
+            title: Text(photo.title!),
           ),
           body: SizedBox.expand(
             child: Hero(
-              tag: photo.tag,
+              tag: photo.tag!,
               child: GridPhotoViewer(photo: photo),
             ),
           ),
@@ -193,10 +191,10 @@ class GridDemoPhotoItem extends StatelessWidget {
       child: GestureDetector(
         onTap: () { showPhoto(context); },
         child: Hero(
-          key: Key(photo.assetName),
-          tag: photo.tag,
+          key: Key(photo.assetName!),
+          tag: photo.tag!,
           child: Image.asset(
-            photo.assetName,
+            photo.assetName!,
             package: photo.assetPackage,
             fit: BoxFit.cover,
           ),
@@ -243,13 +241,11 @@ class GridDemoPhotoItem extends StatelessWidget {
           child: image,
         );
     }
-    assert(tileStyle != null);
-    return null;
   }
 }
 
 class GridListDemo extends StatefulWidget {
-  const GridListDemo({ Key key }) : super(key: key);
+  const GridListDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/grid-list';
 

dev/integration_tests/flutter_gallery/lib/demo/material/icons_demo.dart

@@ -109,7 +109,7 @@ class _IconsDemoCard extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
-    final TextStyle textStyle = theme.textTheme.subtitle1.copyWith(color: theme.textTheme.caption.color);
+    final TextStyle textStyle = theme.textTheme.subtitle1!.copyWith(color: theme.textTheme.caption!.color);
     return Card(
       child: DefaultTextStyle(
         style: textStyle,

dev/integration_tests/flutter_gallery/lib/demo/material/leave_behind_demo.dart

@@ -23,17 +23,17 @@ class LeaveBehindItem implements Comparable<LeaveBehindItem> {
   LeaveBehindItem.from(LeaveBehindItem item)
     : index = item.index, name = item.name, subject = item.subject, body = item.body;
 
-  final int index;
-  final String name;
-  final String subject;
-  final String body;
+  final int? index;
+  final String? name;
+  final String? subject;
+  final String? body;
 
   @override
-  int compareTo(LeaveBehindItem other) => index.compareTo(other.index);
+  int compareTo(LeaveBehindItem other) => index!.compareTo(other.index!);
 }
 
 class LeaveBehindDemo extends StatefulWidget {
-  const LeaveBehindDemo({ Key key }) : super(key: key);
+  const LeaveBehindDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/leave-behind';
 
@@ -44,7 +44,7 @@ class LeaveBehindDemo extends StatefulWidget {
 class LeaveBehindDemoState extends State<LeaveBehindDemo> {
   DismissDirection _dismissDirection = DismissDirection.horizontal;
   bool _confirmDismiss = true;
-  List<LeaveBehindItem> leaveBehindItems;
+  late List<LeaveBehindItem> leaveBehindItems;
 
   void initListItems() {
     leaveBehindItems = List<LeaveBehindItem>.generate(16, (int index) {
@@ -188,12 +188,12 @@ class LeaveBehindDemoState extends State<LeaveBehindDemo> {
 
 class _LeaveBehindListItem extends StatelessWidget {
   const _LeaveBehindListItem({
-    Key key,
-    @required this.item,
-    @required this.onArchive,
-    @required this.onDelete,
-    @required this.dismissDirection,
-    @required this.confirmDismiss,
+    Key? key,
+    required this.item,
+    required this.onArchive,
+    required this.onDelete,
+    required this.dismissDirection,
+    required this.confirmDismiss,
   }) : super(key: key);
 
   final LeaveBehindItem item;
@@ -263,7 +263,7 @@ class _LeaveBehindListItem extends StatelessWidget {
             border: Border(bottom: BorderSide(color: theme.dividerColor)),
           ),
           child: ListTile(
-            title: Text(item.name),
+            title: Text(item.name!),
             subtitle: Text('${item.subject}\n${item.body}'),
             isThreeLine: true,
           ),
@@ -272,7 +272,7 @@ class _LeaveBehindListItem extends StatelessWidget {
     );
   }
 
-  Future<bool> _showConfirmationDialog(BuildContext context, String action) {
+  Future<bool?> _showConfirmationDialog(BuildContext context, String action) {
     return showDialog<bool>(
       context: context,
       barrierDismissible: true,

dev/integration_tests/flutter_gallery/lib/demo/material/list_demo.dart

@@ -21,7 +21,7 @@ enum _MaterialListType {
 }
 
 class ListDemo extends StatefulWidget {
-  const ListDemo({ Key key }) : super(key: key);
+  const ListDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/list';
 
@@ -32,26 +32,26 @@ class ListDemo extends StatefulWidget {
 class _ListDemoState extends State<ListDemo> {
   static final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
 
-  PersistentBottomSheetController<void> _bottomSheet;
-  _MaterialListType _itemType = _MaterialListType.threeLine;
-  bool _dense = false;
-  bool _showAvatars = true;
-  bool _showIcons = false;
-  bool _showDividers = false;
+  PersistentBottomSheetController<void>? _bottomSheet;
+  _MaterialListType? _itemType = _MaterialListType.threeLine;
+  bool? _dense = false;
+  bool? _showAvatars = true;
+  bool? _showIcons = false;
+  bool? _showDividers = false;
   bool _reverseSort = false;
   List<String> items = <String>[
     'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N',
   ];
 
-  void changeItemType(_MaterialListType type) {
+  void changeItemType(_MaterialListType? type) {
     setState(() {
       _itemType = type;
     });
-    _bottomSheet?.setState(() { });
+    _bottomSheet?.setState!(() { });
   }
 
   void _showConfigurationSheet() {
-    final PersistentBottomSheetController<void> bottomSheet = scaffoldKey.currentState.showBottomSheet<void>((BuildContext bottomSheetContext) {
+    final PersistentBottomSheetController<void> bottomSheet = scaffoldKey.currentState!.showBottomSheet<void>((BuildContext bottomSheetContext) {
       return Container(
         decoration: const BoxDecoration(
           border: Border(top: BorderSide(color: Colors.black26)),
@@ -65,7 +65,7 @@ class _ListDemoState extends State<ListDemo> {
                 dense: true,
                 title: const Text('One-line'),
                 trailing: Radio<_MaterialListType>(
-                  value: _showAvatars ? _MaterialListType.oneLineWithAvatar : _MaterialListType.oneLine,
+                  value: _showAvatars! ? _MaterialListType.oneLineWithAvatar : _MaterialListType.oneLine,
                   groupValue: _itemType,
                   onChanged: changeItemType,
                 ),
@@ -99,11 +99,14 @@ class _ListDemoState extends State<ListDemo> {
                 title: const Text('Show avatar'),
                 trailing: Checkbox(
                   value: _showAvatars,
-                  onChanged: (bool value) {
+                  onChanged: (bool? value) {
                     setState(() {
                       _showAvatars = value;
                     });
-                    _bottomSheet?.setState(() { });
+                    final StateSetter? bottomSheetSetState = _bottomSheet?.setState;
+                    if (bottomSheetSetState != null) {
+                      bottomSheetSetState(() { });
+                    }
                   },
                 ),
               ),
@@ -114,11 +117,14 @@ class _ListDemoState extends State<ListDemo> {
                 title: const Text('Show icon'),
                 trailing: Checkbox(
                   value: _showIcons,
-                  onChanged: (bool value) {
+                  onChanged: (bool? value) {
                     setState(() {
                       _showIcons = value;
                     });
-                    _bottomSheet?.setState(() { });
+                    final StateSetter? bottomSheetSetState = _bottomSheet?.setState;
+                    if (bottomSheetSetState != null) {
+                      bottomSheetSetState(() { });
+                    }
                   },
                 ),
               ),
@@ -129,11 +135,14 @@ class _ListDemoState extends State<ListDemo> {
                 title: const Text('Show dividers'),
                 trailing: Checkbox(
                   value: _showDividers,
-                  onChanged: (bool value) {
+                  onChanged: (bool? value) {
                     setState(() {
                       _showDividers = value;
                     });
-                    _bottomSheet?.setState(() { });
+                    final StateSetter? bottomSheetSetState = _bottomSheet?.setState;
+                    if (bottomSheetSetState != null) {
+                      bottomSheetSetState(() { });
+                    }
                   },
                 ),
               ),
@@ -144,11 +153,14 @@ class _ListDemoState extends State<ListDemo> {
                 title: const Text('Dense layout'),
                 trailing: Checkbox(
                   value: _dense,
-                  onChanged: (bool value) {
+                  onChanged: (bool? value) {
                     setState(() {
                       _dense = value;
                     });
-                    _bottomSheet?.setState(() { });
+                    final StateSetter? bottomSheetSetState = _bottomSheet?.setState;
+                    if (bottomSheetSetState != null) {
+                      bottomSheetSetState(() { });
+                    }
                   },
                 ),
               ),
@@ -162,7 +174,7 @@ class _ListDemoState extends State<ListDemo> {
       _bottomSheet = bottomSheet;
     });
 
-    _bottomSheet.closed.whenComplete(() {
+    _bottomSheet?.closed.whenComplete(() {
       if (mounted) {
         setState(() {
           _bottomSheet = null;
@@ -172,7 +184,7 @@ class _ListDemoState extends State<ListDemo> {
   }
 
   Widget buildListTile(BuildContext context, String item) {
-    Widget secondary;
+    Widget? secondary;
     if (_itemType == _MaterialListType.twoLine) {
       secondary = const Text('Additional item information.');
     } else if (_itemType == _MaterialListType.threeLine) {
@@ -184,18 +196,18 @@ class _ListDemoState extends State<ListDemo> {
       child: ListTile(
         isThreeLine: _itemType == _MaterialListType.threeLine,
         dense: _dense,
-        leading: _showAvatars ? ExcludeSemantics(child: CircleAvatar(child: Text(item))) : null,
+        leading: _showAvatars != null ? ExcludeSemantics(child: CircleAvatar(child: Text(item))) : null,
         title: Text('This item represents $item.'),
         subtitle: secondary,
-        trailing: _showIcons ? Icon(Icons.info, color: Theme.of(context).disabledColor) : null,
+        trailing: _showIcons != null ? Icon(Icons.info, color: Theme.of(context).disabledColor) : null,
       ),
     );
   }
 
   @override
   Widget build(BuildContext context) {
-    final String layoutText = _dense ? ' \u2013 Dense' : '';
-    String itemTypeText;
+    final String layoutText = _dense != null ? ' \u2013 Dense' : '';
+    String? itemTypeText;
     switch (_itemType) {
       case _MaterialListType.oneLine:
       case _MaterialListType.oneLineWithAvatar:
@@ -207,10 +219,12 @@ class _ListDemoState extends State<ListDemo> {
       case _MaterialListType.threeLine:
         itemTypeText = 'Three-line';
         break;
+      default:
+        break;
     }
 
     Iterable<Widget> listTiles = items.map<Widget>((String item) => buildListTile(context, item));
-    if (_showDividers)
+    if (_showDividers != null)
       listTiles = ListTile.divideTiles(context: context, tiles: listTiles);
 
     return Scaffold(
@@ -242,7 +256,7 @@ class _ListDemoState extends State<ListDemo> {
       ),
       body: Scrollbar(
         child: ListView(
-          padding: EdgeInsets.symmetric(vertical: _dense ? 4.0 : 8.0),
+          padding: EdgeInsets.symmetric(vertical: _dense != null ? 4.0 : 8.0),
           children: listTiles.toList(),
         ),
       ),

dev/integration_tests/flutter_gallery/lib/demo/material/menu_demo.dart

@@ -7,7 +7,7 @@ import 'package:flutter/material.dart';
 import '../../gallery/demo.dart';
 
 class MenuDemo extends StatefulWidget {
-  const MenuDemo({ Key key }) : super(key: key);
+  const MenuDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/menu';
 
@@ -20,13 +20,13 @@ class MenuDemoState extends State<MenuDemo> {
   final String _simpleValue1 = 'Menu item value one';
   final String _simpleValue2 = 'Menu item value two';
   final String _simpleValue3 = 'Menu item value three';
-  String _simpleValue;
+  String? _simpleValue;
 
   final String _checkedValue1 = 'One';
   final String _checkedValue2 = 'Two';
   final String _checkedValue3 = 'Free';
   final String _checkedValue4 = 'Four';
-  List<String> _checkedValues;
+  late List<String> _checkedValues;
 
   @override
   void initState() {
@@ -165,7 +165,7 @@ class MenuDemoState extends State<MenuDemo> {
               onSelected: showMenuSelection,
               child: ListTile(
                 title: const Text('An item with a simple menu'),
-                subtitle: Text(_simpleValue),
+                subtitle: Text(_simpleValue!),
               ),
               itemBuilder: (BuildContext context) => <PopupMenuItem<String>>[
                 PopupMenuItem<String>(

dev/integration_tests/flutter_gallery/lib/demo/material/overscroll_demo.dart

@@ -11,7 +11,7 @@ import '../../gallery/demo.dart';
 enum IndicatorType { overscroll, refresh }
 
 class OverscrollDemo extends StatefulWidget {
-  const OverscrollDemo({ Key key }) : super(key: key);
+  const OverscrollDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/overscroll';
 
@@ -28,13 +28,13 @@ class OverscrollDemoState extends State<OverscrollDemo> {
   Future<void> _handleRefresh() {
     final Completer<void> completer = Completer<void>();
     Timer(const Duration(seconds: 3), () { completer.complete(); });
-    return completer.future.then<void>((_) {
+    return completer.future.then((_) {
       ScaffoldMessenger.of(context).showSnackBar(SnackBar(
         content: const Text('Refresh complete'),
         action: SnackBarAction(
           label: 'RETRY',
           onPressed: () {
-            _refreshIndicatorKey.currentState.show();
+            _refreshIndicatorKey.currentState!.show();
           },
         ),
       ));
@@ -52,7 +52,7 @@ class OverscrollDemoState extends State<OverscrollDemo> {
             icon: const Icon(Icons.refresh),
             tooltip: 'Refresh',
             onPressed: () {
-              _refreshIndicatorKey.currentState.show();
+              _refreshIndicatorKey.currentState!.show();
             },
           ),
         ],

dev/integration_tests/flutter_gallery/lib/demo/material/page_selector_demo.dart

@@ -9,17 +9,17 @@ import '../../gallery/demo.dart';
 class _PageSelector extends StatelessWidget {
   const _PageSelector({ this.icons });
 
-  final List<Icon> icons;
+  final List<Icon>? icons;
 
   void _handleArrowButtonPress(BuildContext context, int delta) {
-    final TabController controller = DefaultTabController.of(context);
+    final TabController controller = DefaultTabController.of(context)!;
     if (!controller.indexIsChanging)
-      controller.animateTo((controller.index + delta).clamp(0, icons.length - 1) as int);
+      controller.animateTo((controller.index + delta).clamp(0, icons!.length - 1));
   }
 
   @override
   Widget build(BuildContext context) {
-    final TabController controller = DefaultTabController.of(context);
+    final TabController? controller = DefaultTabController.of(context);
     final Color color = Theme.of(context).accentColor;
     return SafeArea(
       top: false,
@@ -54,7 +54,7 @@ class _PageSelector extends StatelessWidget {
                 color: color,
               ),
               child: TabBarView(
-                children: icons.map<Widget>((Icon icon) {
+                children: icons!.map<Widget>((Icon icon) {
                   return Container(
                     padding: const EdgeInsets.all(12.0),
                     child: Card(

dev/integration_tests/flutter_gallery/lib/demo/material/persistent_bottom_sheet_demo.dart

@@ -16,7 +16,7 @@ class PersistentBottomSheetDemo extends StatefulWidget {
 class _PersistentBottomSheetDemoState extends State<PersistentBottomSheetDemo> {
   final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
 
-  VoidCallback _showBottomSheetCallback;
+  VoidCallback? _showBottomSheetCallback;
 
   @override
   void initState() {
@@ -28,7 +28,7 @@ class _PersistentBottomSheetDemoState extends State<PersistentBottomSheetDemo> {
     setState(() { // disable the button
       _showBottomSheetCallback = null;
     });
-    _scaffoldKey.currentState.showBottomSheet<void>((BuildContext context) {
+    _scaffoldKey.currentState!.showBottomSheet<void>((BuildContext context) {
       final ThemeData themeData = Theme.of(context);
       return Container(
         decoration: BoxDecoration(

dev/integration_tests/flutter_gallery/lib/demo/material/progress_indicator_demo.dart

@@ -14,8 +14,8 @@ class ProgressIndicatorDemo extends StatefulWidget {
 }
 
 class _ProgressIndicatorDemoState extends State<ProgressIndicatorDemo> with SingleTickerProviderStateMixin {
-  AnimationController _controller;
-  Animation<double> _animation;
+  late AnimationController _controller;
+  late Animation<double> _animation;
 
   @override
   void initState() {
@@ -64,7 +64,7 @@ class _ProgressIndicatorDemoState extends State<ProgressIndicatorDemo> with Sing
     });
   }
 
-  Widget _buildIndicators(BuildContext context, Widget child) {
+  Widget _buildIndicators(BuildContext context, Widget? child) {
     final List<Widget> indicators = <Widget>[
       const SizedBox(
         width: 200.0,
@@ -109,7 +109,7 @@ class _ProgressIndicatorDemoState extends State<ProgressIndicatorDemo> with Sing
       body: Center(
         child: SingleChildScrollView(
           child: DefaultTextStyle(
-            style: Theme.of(context).textTheme.headline6,
+            style: Theme.of(context).textTheme.headline6!,
             child: GestureDetector(
               onTap: _handleTap,
               behavior: HitTestBehavior.opaque,

dev/integration_tests/flutter_gallery/lib/demo/material/reorderable_list_demo.dart

@@ -20,7 +20,7 @@ enum _ReorderableListType {
 }
 
 class ReorderableListDemo extends StatefulWidget {
-  const ReorderableListDemo({ Key key }) : super(key: key);
+  const ReorderableListDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/reorderable-list';
 
@@ -33,38 +33,38 @@ class _ListItem {
 
   final String value;
 
-  bool checkState;
+  bool? checkState;
 }
 
 class _ListDemoState extends State<ReorderableListDemo> {
   static final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
 
-  PersistentBottomSheetController<void> _bottomSheet;
-  _ReorderableListType _itemType = _ReorderableListType.threeLine;
-  bool _reverse = false;
+  PersistentBottomSheetController<void>? _bottomSheet;
+  _ReorderableListType? _itemType = _ReorderableListType.threeLine;
+  bool? _reverse = false;
   bool _reverseSort = false;
   final List<_ListItem> _items = <String>[
     'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N',
   ].map<_ListItem>((String item) => _ListItem(item, false)).toList();
 
-  void changeItemType(_ReorderableListType type) {
+  void changeItemType(_ReorderableListType? type) {
     setState(() {
       _itemType = type;
     });
     // Rebuild the bottom sheet to reflect the selected list view.
-    _bottomSheet?.setState(() {
+    _bottomSheet?.setState!(() {
       // Trigger a rebuild.
     });
     // Close the bottom sheet to give the user a clear view of the list.
     _bottomSheet?.close();
   }
 
-  void changeReverse(bool newValue) {
+  void changeReverse(bool? newValue) {
     setState(() {
       _reverse = newValue;
     });
     // Rebuild the bottom sheet to reflect the selected list view.
-    _bottomSheet?.setState(() {
+    _bottomSheet?.setState!(() {
       // Trigger a rebuild.
     });
     // Close the bottom sheet to give the user a clear view of the list.
@@ -73,7 +73,7 @@ class _ListDemoState extends State<ReorderableListDemo> {
 
   void _showConfigurationSheet() {
     setState(() {
-      _bottomSheet = scaffoldKey.currentState.showBottomSheet<void>((BuildContext bottomSheetContext) {
+      _bottomSheet = scaffoldKey.currentState!.showBottomSheet<void>((BuildContext bottomSheetContext) {
         return DecoratedBox(
           decoration: const BoxDecoration(
             border: Border(top: BorderSide(color: Colors.black26)),
@@ -115,7 +115,7 @@ class _ListDemoState extends State<ReorderableListDemo> {
       });
 
       // Garbage collect the bottom sheet when it closes.
-      _bottomSheet.closed.whenComplete(() {
+      _bottomSheet?.closed.whenComplete(() {
         if (mounted) {
           setState(() {
             _bottomSheet = null;
@@ -129,14 +129,14 @@ class _ListDemoState extends State<ReorderableListDemo> {
     const Widget secondary = Text(
       'Even more additional list item information appears on line three.',
     );
-    Widget listTile;
+    late Widget listTile;
     switch (_itemType) {
       case _ReorderableListType.threeLine:
         listTile = CheckboxListTile(
           key: Key(item.value),
           isThreeLine: true,
           value: item.checkState ?? false,
-          onChanged: (bool newValue) {
+          onChanged: (bool? newValue) {
             setState(() {
               item.checkState = newValue;
             });
@@ -157,6 +157,11 @@ class _ListDemoState extends State<ReorderableListDemo> {
           ),
         );
         break;
+      default:
+        listTile = Container(
+          key: Key(item.value),
+        );
+        break;
     }
 
     return listTile;
@@ -210,7 +215,7 @@ class _ListDemoState extends State<ReorderableListDemo> {
                   child: Text('Header of the list', style: Theme.of(context).textTheme.headline5))
               : null,
           onReorder: _onReorder,
-          reverse: _reverse,
+          reverse: _reverse!,
           scrollDirection: _itemType == _ReorderableListType.horizontalAvatar ? Axis.horizontal : Axis.vertical,
           padding: const EdgeInsets.symmetric(vertical: 8.0),
           children: _items.map<Widget>(buildListTile).toList(),

dev/integration_tests/flutter_gallery/lib/demo/material/scrollable_tabs_demo.dart

@@ -14,8 +14,8 @@ enum TabsDemoStyle {
 
 class _Page {
   const _Page({ this.icon, this.text });
-  final IconData icon;
-  final String text;
+  final IconData? icon;
+  final String? text;
 }
 
 const List<_Page> _allPages = <_Page>[
@@ -43,7 +43,7 @@ class ScrollableTabsDemo extends StatefulWidget {
 }
 
 class ScrollableTabsDemoState extends State<ScrollableTabsDemo> with SingleTickerProviderStateMixin {
-  TabController _controller;
+  TabController? _controller;
   TabsDemoStyle _demoStyle = TabsDemoStyle.iconsAndText;
   bool _customIndicator = false;
 
@@ -55,7 +55,7 @@ class ScrollableTabsDemoState extends State<ScrollableTabsDemo> with SingleTicke
 
   @override
   void dispose() {
-    _controller.dispose();
+    _controller!.dispose();
     super.dispose();
   }
 
@@ -65,7 +65,7 @@ class ScrollableTabsDemoState extends State<ScrollableTabsDemo> with SingleTicke
     });
   }
 
-  Decoration getIndicator() {
+  Decoration? getIndicator() {
     if (!_customIndicator)
       return const UnderlineTabIndicator();
 
@@ -117,7 +117,6 @@ class ScrollableTabsDemoState extends State<ScrollableTabsDemo> with SingleTicke
           ),
         );
     }
-    return null;
   }
 
   @override
@@ -161,7 +160,6 @@ class ScrollableTabsDemoState extends State<ScrollableTabsDemo> with SingleTicke
           isScrollable: true,
           indicator: getIndicator(),
           tabs: _allPages.map<Tab>((_Page page) {
-            assert(_demoStyle != null);
             switch (_demoStyle) {
               case TabsDemoStyle.iconsAndText:
                 return Tab(text: page.text, icon: Icon(page.icon));
@@ -169,9 +167,10 @@ class ScrollableTabsDemoState extends State<ScrollableTabsDemo> with SingleTicke
                 return Tab(icon: Icon(page.icon));
               case TabsDemoStyle.textOnly:
                 return Tab(text: page.text);
+              default:
+                return Tab();
             }
-            return null;
-          }).toList(),
+          }).toList()
         ),
       ),
       body: TabBarView(

dev/integration_tests/flutter_gallery/lib/demo/material/search_demo.dart

@@ -17,7 +17,7 @@ class _SearchDemoState extends State<SearchDemo> {
   final _SearchDemoSearchDelegate _delegate = _SearchDemoSearchDelegate();
   final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
 
-  int _lastIntegerSelected;
+  int? _lastIntegerSelected;
 
   @override
   Widget build(BuildContext context) {
@@ -32,7 +32,7 @@ class _SearchDemoState extends State<SearchDemo> {
             progress: _delegate.transitionAnimation,
           ),
           onPressed: () {
-            _scaffoldKey.currentState.openDrawer();
+            _scaffoldKey.currentState!.openDrawer();
           },
         ),
         title: const Text('Numbers'),
@@ -41,7 +41,7 @@ class _SearchDemoState extends State<SearchDemo> {
             tooltip: 'Search',
             icon: const Icon(Icons.search),
             onPressed: () async {
-              final int selected = await showSearch<int>(
+              final int? selected = await showSearch<int?>(
                 context: context,
                 delegate: _delegate,
               );
@@ -98,7 +98,7 @@ class _SearchDemoState extends State<SearchDemo> {
       floatingActionButton: FloatingActionButton.extended(
         tooltip: 'Back', // Tests depend on this label to exit the demo.
         onPressed: () {
-          Navigator.of(context).pop();
+          Navigator.of(context)!.pop();
         },
         label: const Text('Close demo'),
         icon: const Icon(Icons.close),
@@ -133,7 +133,7 @@ class _SearchDemoState extends State<SearchDemo> {
   }
 }
 
-class _SearchDemoSearchDelegate extends SearchDelegate<int> {
+class _SearchDemoSearchDelegate extends SearchDelegate<int?> {
   final List<int> _data = List<int>.generate(100001, (int i) => i).reversed.toList();
   final List<int> _history = <int>[42607, 85604, 66374, 44, 174];
 
@@ -170,7 +170,7 @@ class _SearchDemoSearchDelegate extends SearchDelegate<int> {
 
   @override
   Widget buildResults(BuildContext context) {
-    final int searched = int.tryParse(query);
+    final int? searched = int.tryParse(query);
     if (searched == null || !_data.contains(searched)) {
       return Center(
         child: Text(
@@ -228,26 +228,26 @@ class _SearchDemoSearchDelegate extends SearchDelegate<int> {
 class _ResultCard extends StatelessWidget {
   const _ResultCard({this.integer, this.title, this.searchDelegate});
 
-  final int integer;
-  final String title;
-  final SearchDelegate<int> searchDelegate;
+  final int? integer;
+  final String? title;
+  final SearchDelegate<int?>? searchDelegate;
 
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
     return GestureDetector(
       onTap: () {
-        searchDelegate.close(context, integer);
+        searchDelegate!.close(context, integer);
       },
       child: Card(
         child: Padding(
           padding: const EdgeInsets.all(8.0),
           child: Column(
             children: <Widget>[
-              Text(title),
+              Text(title!),
               Text(
                 '$integer',
-                style: theme.textTheme.headline5.copyWith(fontSize: 72.0),
+                style: theme.textTheme.headline5!.copyWith(fontSize: 72.0),
               ),
             ],
           ),
@@ -260,33 +260,33 @@ class _ResultCard extends StatelessWidget {
 class _SuggestionList extends StatelessWidget {
   const _SuggestionList({this.suggestions, this.query, this.onSelected});
 
-  final List<String> suggestions;
-  final String query;
-  final ValueChanged<String> onSelected;
+  final List<String>? suggestions;
+  final String? query;
+  final ValueChanged<String>? onSelected;
 
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
     return ListView.builder(
-      itemCount: suggestions.length,
+      itemCount: suggestions!.length,
       itemBuilder: (BuildContext context, int i) {
-        final String suggestion = suggestions[i];
+        final String suggestion = suggestions![i];
         return ListTile(
-          leading: query.isEmpty ? const Icon(Icons.history) : const Icon(null),
+          leading: query!.isEmpty ? const Icon(Icons.history) : const Icon(null),
           title: RichText(
             text: TextSpan(
-              text: suggestion.substring(0, query.length),
-              style: theme.textTheme.subtitle1.copyWith(fontWeight: FontWeight.bold),
+              text: suggestion.substring(0, query!.length),
+              style: theme.textTheme.subtitle1!.copyWith(fontWeight: FontWeight.bold),
               children: <TextSpan>[
                 TextSpan(
-                  text: suggestion.substring(query.length),
+                  text: suggestion.substring(query!.length),
                   style: theme.textTheme.subtitle1,
                 ),
               ],
             ),
           ),
           onTap: () {
-            onSelected(suggestion);
+            onSelected!(suggestion);
           },
         );
       },

dev/integration_tests/flutter_gallery/lib/demo/material/selection_controls_demo.dart

@@ -67,13 +67,13 @@ class _SelectionControlsDemoState extends State<SelectionControlsDemo> {
     );
   }
 
-  bool checkboxValueA = true;
-  bool checkboxValueB = false;
-  bool checkboxValueC;
-  int radioValue = 0;
+  bool? checkboxValueA = true;
+  bool? checkboxValueB = false;
+  bool? checkboxValueC;
+  int? radioValue = 0;
   bool switchValue = false;
 
-  void handleRadioValueChanged(int value) {
+  void handleRadioValueChanged(int? value) {
     setState(() {
       radioValue = value;
     });
@@ -92,7 +92,7 @@ class _SelectionControlsDemoState extends State<SelectionControlsDemo> {
                 label: 'Checkbox A',
                 child: Checkbox(
                   value: checkboxValueA,
-                  onChanged: (bool value) {
+                  onChanged: (bool? value) {
                     setState(() {
                       checkboxValueA = value;
                     });
@@ -103,7 +103,7 @@ class _SelectionControlsDemoState extends State<SelectionControlsDemo> {
                 label: 'Checkbox B',
                 child: Checkbox(
                   value: checkboxValueB,
-                  onChanged: (bool value) {
+                  onChanged: (bool? value) {
                     setState(() {
                       checkboxValueB = value;
                     });
@@ -115,7 +115,7 @@ class _SelectionControlsDemoState extends State<SelectionControlsDemo> {
                 child: Checkbox(
                   value: checkboxValueC,
                   tristate: true,
-                  onChanged: (bool value) {
+                  onChanged: (bool? value) {
                     setState(() {
                       checkboxValueC = value;
                     });

dev/integration_tests/flutter_gallery/lib/demo/material/slider_demo.dart

@@ -61,15 +61,15 @@ class _CustomRangeThumbShape extends RangeSliderThumbShape {
   void paint(
     PaintingContext context,
     Offset center, {
-    @required Animation<double> activationAnimation,
-    @required Animation<double> enableAnimation,
+    required Animation<double> activationAnimation,
+    required Animation<double> enableAnimation,
     bool isDiscrete = false,
     bool isEnabled = false,
-    bool isOnTop,
-    @required SliderThemeData sliderTheme,
-    TextDirection textDirection,
-    Thumb thumb,
-    bool isPressed,
+    bool? isOnTop,
+    required SliderThemeData sliderTheme,
+    TextDirection? textDirection,
+    Thumb? thumb,
+    bool? isPressed,
   }) {
     final Canvas canvas = context.canvas;
     final ColorTween colorTween = ColorTween(
@@ -78,7 +78,7 @@ class _CustomRangeThumbShape extends RangeSliderThumbShape {
     );
 
     final double size = _thumbSize * sizeTween.evaluate(enableAnimation);
-    Path thumbPath;
+    late Path thumbPath;
     switch (textDirection) {
       case TextDirection.rtl:
         switch (thumb) {
@@ -88,6 +88,8 @@ class _CustomRangeThumbShape extends RangeSliderThumbShape {
           case Thumb.end:
             thumbPath = _leftTriangle(size, center);
             break;
+          default:
+            break;
         }
         break;
       case TextDirection.ltr:
@@ -98,10 +100,14 @@ class _CustomRangeThumbShape extends RangeSliderThumbShape {
           case Thumb.end:
             thumbPath = _rightTriangle(size, center);
             break;
+          default:
+            break;
         }
         break;
+      default:
+        break;
     }
-    canvas.drawPath(thumbPath, Paint()..color = colorTween.evaluate(enableAnimation));
+    canvas.drawPath(thumbPath, Paint()..color = colorTween.evaluate(enableAnimation)!);
   }
 }
 
@@ -123,16 +129,16 @@ class _CustomThumbShape extends SliderComponentShape {
   void paint(
     PaintingContext context,
     Offset thumbCenter, {
-    Animation<double> activationAnimation,
-    Animation<double> enableAnimation,
-    bool isDiscrete,
-    TextPainter labelPainter,
-    RenderBox parentBox,
-    SliderThemeData sliderTheme,
-    TextDirection textDirection,
-    double value,
-    double textScaleFactor,
-    Size sizeWithOverflow,
+    Animation<double>? activationAnimation,
+    required Animation<double> enableAnimation,
+    bool? isDiscrete,
+    TextPainter? labelPainter,
+    RenderBox? parentBox,
+    required SliderThemeData sliderTheme,
+    TextDirection? textDirection,
+    double? value,
+    double? textScaleFactor,
+    Size? sizeWithOverflow,
   }) {
     final Canvas canvas = context.canvas;
     final ColorTween colorTween = ColorTween(
@@ -141,7 +147,7 @@ class _CustomThumbShape extends SliderComponentShape {
     );
     final double size = _thumbSize * sizeTween.evaluate(enableAnimation);
     final Path thumbPath = _downTriangle(size, thumbCenter);
-    canvas.drawPath(thumbPath, Paint()..color = colorTween.evaluate(enableAnimation));
+    canvas.drawPath(thumbPath, Paint()..color = colorTween.evaluate(enableAnimation)!);
   }
 }
 
@@ -164,16 +170,16 @@ class _CustomValueIndicatorShape extends SliderComponentShape {
   void paint(
     PaintingContext context,
     Offset thumbCenter, {
-    Animation<double> activationAnimation,
-    Animation<double> enableAnimation,
-    bool isDiscrete,
-    TextPainter labelPainter,
-    RenderBox parentBox,
-    SliderThemeData sliderTheme,
-    TextDirection textDirection,
-    double value,
-    double textScaleFactor,
-    Size sizeWithOverflow,
+    required Animation<double> activationAnimation,
+    required Animation<double> enableAnimation,
+    bool? isDiscrete,
+    required TextPainter labelPainter,
+    RenderBox? parentBox,
+    required SliderThemeData sliderTheme,
+    TextDirection? textDirection,
+    double? value,
+    double? textScaleFactor,
+    Size? sizeWithOverflow,
   }) {
     final Canvas canvas = context.canvas;
     final ColorTween enableColor = ColorTween(
@@ -187,7 +193,7 @@ class _CustomValueIndicatorShape extends SliderComponentShape {
     final double size = _indicatorSize * sizeTween.evaluate(enableAnimation);
     final Offset slideUpOffset = Offset(0.0, -slideUpTween.evaluate(activationAnimation));
     final Path thumbPath = _upTriangle(size, thumbCenter + slideUpOffset);
-    final Color paintColor = enableColor.evaluate(enableAnimation).withAlpha((255.0 * activationAnimation.value).round());
+    final Color paintColor = enableColor.evaluate(enableAnimation)!.withAlpha((255.0 * activationAnimation.value).round());
     canvas.drawPath(
       thumbPath,
       Paint()..color = paintColor,
@@ -259,10 +265,10 @@ class _SlidersState extends State<_Sliders> {
                   child: TextField(
                     textAlign: TextAlign.center,
                     onSubmitted: (String value) {
-                      final double newValue = double.tryParse(value);
+                      final double? newValue = double.tryParse(value);
                       if (newValue != null && newValue != _continuousValue) {
                         setState(() {
-                          _continuousValue = newValue.clamp(0.0, 100.0) as double;
+                          _continuousValue = newValue.clamp(0.0, 100.0);
                         });
                       }
                     },
@@ -326,7 +332,7 @@ class _SlidersState extends State<_Sliders> {
                   valueIndicatorColor: Colors.deepPurpleAccent,
                   thumbShape: _CustomThumbShape(),
                   valueIndicatorShape: _CustomValueIndicatorShape(),
-                  valueIndicatorTextStyle: theme.accentTextTheme.bodyText1.copyWith(color: theme.colorScheme.onSurface),
+                  valueIndicatorTextStyle: theme.accentTextTheme.bodyText1!.copyWith(color: theme.colorScheme.onSurface),
                 ),
                 child: Slider(
                   value: _discreteCustomValue,

dev/integration_tests/flutter_gallery/lib/demo/material/snack_bar_demo.dart

@@ -19,7 +19,7 @@ const String _text3 =
   'By default snackbars automatically disappear after a few seconds ';
 
 class SnackBarDemo extends StatefulWidget {
-  const SnackBarDemo({ Key key }) : super(key: key);
+  const SnackBarDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/snack-bar';
 

dev/integration_tests/flutter_gallery/lib/demo/material/tabs_demo.dart

@@ -13,17 +13,17 @@ const String _kGalleryAssetsPackage = 'flutter_gallery_assets';
 
 class _Page {
   _Page({ this.label });
-  final String label;
-  String get id => label.characters.first;
+  final String? label;
+  String get id => label!.characters.first;
   @override
   String toString() => '$runtimeType("$label")';
 }
 
 class _CardData {
   const _CardData({ this.title, this.imageAsset, this.imageAssetPackage });
-  final String title;
-  final String imageAsset;
-  final String imageAssetPackage;
+  final String? title;
+  final String? imageAsset;
+  final String? imageAssetPackage;
 }
 
 final Map<_Page, List<_CardData>> _allPages = <_Page, List<_CardData>>{
@@ -97,8 +97,8 @@ class _CardDataItem extends StatelessWidget {
   const _CardDataItem({ this.page, this.data });
 
   static const double height = 272.0;
-  final _Page page;
-  final _CardData data;
+  final _Page? page;
+  final _CardData? data;
 
   @override
   Widget build(BuildContext context) {
@@ -110,23 +110,23 @@ class _CardDataItem extends StatelessWidget {
           mainAxisAlignment: MainAxisAlignment.start,
           children: <Widget>[
             Align(
-              alignment: page.id == 'H'
+              alignment: page!.id == 'H'
                 ? Alignment.centerLeft
                 : Alignment.centerRight,
-              child: CircleAvatar(child: Text(page.id)),
+              child: CircleAvatar(child: Text(page!.id)),
             ),
             SizedBox(
               width: 144.0,
               height: 144.0,
               child: Image.asset(
-                data.imageAsset,
-                package: data.imageAssetPackage,
+                data!.imageAsset!,
+                package: data!.imageAssetPackage,
                 fit: BoxFit.contain,
               ),
             ),
             Center(
               child: Text(
-                data.title,
+                data!.title!,
                 style: Theme.of(context).textTheme.headline6,
               ),
             ),
@@ -187,7 +187,7 @@ class TabsDemo extends StatelessWidget {
                             itemExtent: _CardDataItem.height,
                             delegate: SliverChildBuilderDelegate(
                               (BuildContext context, int index) {
-                                final _CardData data = _allPages[page][index];
+                                final _CardData data = _allPages[page]![index];
                                 return Padding(
                                   padding: const EdgeInsets.symmetric(
                                     vertical: 8.0,
@@ -198,7 +198,7 @@ class TabsDemo extends StatelessWidget {
                                   ),
                                 );
                               },
-                              childCount: _allPages[page].length,
+                              childCount: _allPages[page]!.length,
                             ),
                           ),
                         ),

dev/integration_tests/flutter_gallery/lib/demo/material/tabs_fab_demo.dart

@@ -15,13 +15,13 @@ const String _explanatoryText =
 class _Page {
   _Page({ this.label, this.colors, this.icon });
 
-  final String label;
-  final MaterialColor colors;
-  final IconData icon;
+  final String? label;
+  final MaterialColor? colors;
+  final IconData? icon;
 
-  Color get labelColor => colors != null ? colors.shade300 : Colors.grey.shade300;
+  Color get labelColor => colors != null ? colors!.shade300 : Colors.grey.shade300;
   bool get fabDefined => colors != null && icon != null;
-  Color get fabColor => colors.shade400;
+  Color get fabColor => colors!.shade400;
   Icon get fabIcon => Icon(icon);
   Key get fabKey => ValueKey<Color>(fabColor);
 }
@@ -44,32 +44,32 @@ class TabsFabDemo extends StatefulWidget {
 class _TabsFabDemoState extends State<TabsFabDemo> with SingleTickerProviderStateMixin {
   final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
 
-  TabController _controller;
-  _Page _selectedPage;
+  TabController? _controller;
+  late _Page _selectedPage;
   bool _extendedButtons = false;
 
   @override
   void initState() {
     super.initState();
     _controller = TabController(vsync: this, length: _allPages.length);
-    _controller.addListener(_handleTabSelection);
+    _controller!.addListener(_handleTabSelection);
     _selectedPage = _allPages[0];
   }
 
   @override
   void dispose() {
-    _controller.dispose();
+    _controller!.dispose();
     super.dispose();
   }
 
   void _handleTabSelection() {
     setState(() {
-      _selectedPage = _allPages[_controller.index];
+      _selectedPage = _allPages[_controller!.index];
     });
   }
 
   void _showExplanatoryText() {
-    _scaffoldKey.currentState.showBottomSheet<void>((BuildContext context) {
+    _scaffoldKey.currentState!.showBottomSheet<void>((BuildContext context) {
       return Container(
         decoration: BoxDecoration(
           border: Border(top: BorderSide(color: Theme.of(context).dividerColor))
@@ -86,11 +86,11 @@ class _TabsFabDemoState extends State<TabsFabDemo> with SingleTickerProviderStat
     return Builder(
       builder: (BuildContext context) {
         return Container(
-          key: ValueKey<String>(page.label),
+          key: ValueKey<String?>(page.label),
           padding: const EdgeInsets.fromLTRB(48.0, 48.0, 48.0, 96.0),
           child: Card(
             child: Center(
-              child: Text(page.label,
+              child: Text(page.label!,
                 style: TextStyle(
                   color: page.labelColor,
                   fontSize: 32.0,
@@ -104,7 +104,7 @@ class _TabsFabDemoState extends State<TabsFabDemo> with SingleTickerProviderStat
     );
   }
 
-  Widget buildFloatingActionButton(_Page page) {
+  Widget? buildFloatingActionButton(_Page page) {
     if (!page.fabDefined)
       return null;
 
@@ -114,7 +114,7 @@ class _TabsFabDemoState extends State<TabsFabDemo> with SingleTickerProviderStat
         tooltip: 'Show explanation',
         backgroundColor: page.fabColor,
         icon: page.fabIcon,
-        label: Text(page.label.toUpperCase()),
+        label: Text(page.label!.toUpperCase()),
         onPressed: _showExplanatoryText,
       );
     }
@@ -136,7 +136,7 @@ class _TabsFabDemoState extends State<TabsFabDemo> with SingleTickerProviderStat
         title: const Text('FAB per tab'),
         bottom: TabBar(
           controller: _controller,
-          tabs: _allPages.map<Widget>((_Page page) => Tab(text: page.label.toUpperCase())).toList(),
+          tabs: _allPages.map<Widget>((_Page page) => Tab(text: page.label!.toUpperCase())).toList(),
         ),
         actions: <Widget>[
           MaterialDemoDocumentationButton(TabsFabDemo.routeName),

dev/integration_tests/flutter_gallery/lib/demo/material/text_form_field_demo.dart

@@ -9,7 +9,7 @@ import 'package:flutter/gestures.dart' show DragStartBehavior;
 import '../../gallery/demo.dart';
 
 class TextFormFieldDemo extends StatefulWidget {
-  const TextFormFieldDemo({ Key key }) : super(key: key);
+  const TextFormFieldDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/material/text-form-field';
 
@@ -18,9 +18,9 @@ class TextFormFieldDemo extends StatefulWidget {
 }
 
 class PersonData {
-  String name = '';
-  String phoneNumber = '';
-  String email = '';
+  String? name = '';
+  String? phoneNumber = '';
+  String? email = '';
   String password = '';
 }
 
@@ -35,13 +35,13 @@ class PasswordField extends StatefulWidget {
     this.onFieldSubmitted,
   });
 
-  final Key fieldKey;
-  final String hintText;
-  final String labelText;
-  final String helperText;
-  final FormFieldSetter<String> onSaved;
-  final FormFieldValidator<String> validator;
-  final ValueChanged<String> onFieldSubmitted;
+  final Key? fieldKey;
+  final String? hintText;
+  final String? labelText;
+  final String? helperText;
+  final FormFieldSetter<String>? onSaved;
+  final FormFieldValidator<String>? validator;
+  final ValueChanged<String>? onFieldSubmitted;
 
   @override
   _PasswordFieldState createState() => _PasswordFieldState();
@@ -99,7 +99,7 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
   final GlobalKey<FormFieldState<String>> _passwordFieldKey = GlobalKey<FormFieldState<String>>();
   final _UsNumberTextInputFormatter _phoneNumberFormatter = _UsNumberTextInputFormatter();
   void _handleSubmitted() {
-    final FormState form = _formKey.currentState;
+    final FormState form = _formKey.currentState!;
     if (!form.validate()) {
       _autovalidateMode = AutovalidateMode.always; // Start validating on every change.
       showInSnackBar('Please fix the errors in red before submitting.');
@@ -109,9 +109,9 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
     }
   }
 
-  String _validateName(String value) {
+  String? _validateName(String? value) {
     _formWasEdited = true;
-    if (value.isEmpty)
+    if (value!.isEmpty)
       return 'Name is required.';
     final RegExp nameExp = RegExp(r'^[A-Za-z ]+$');
     if (!nameExp.hasMatch(value))
@@ -119,18 +119,18 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
     return null;
   }
 
-  String _validatePhoneNumber(String value) {
+  String? _validatePhoneNumber(String? value) {
     _formWasEdited = true;
     final RegExp phoneExp = RegExp(r'^\(\d\d\d\) \d\d\d\-\d\d\d\d$');
-    if (!phoneExp.hasMatch(value))
+    if (!phoneExp.hasMatch(value!))
       return '(###) ###-#### - Enter a US phone number.';
     return null;
   }
 
-  String _validatePassword(String value) {
+  String? _validatePassword(String? value) {
     _formWasEdited = true;
-    final FormFieldState<String> passwordField = _passwordFieldKey.currentState;
-    if (passwordField.value == null || passwordField.value.isEmpty)
+    final FormFieldState<String> passwordField = _passwordFieldKey.currentState!;
+    if (passwordField.value == null || passwordField.value!.isEmpty)
       return 'Please enter a password.';
     if (passwordField.value != value)
       return "The passwords don't match";
@@ -138,11 +138,11 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
   }
 
   Future<bool> _warnUserAboutInvalidData() async {
-    final FormState form = _formKey.currentState;
+    final FormState? form = _formKey.currentState;
     if (form == null || !_formWasEdited || form.validate())
       return true;
 
-    return await showDialog<bool>(
+    return await (showDialog<bool>(
       context: context,
       builder: (BuildContext context) {
         return AlertDialog(
@@ -151,16 +151,16 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
           actions: <Widget> [
             TextButton(
               child: const Text('YES'),
-              onPressed: () { Navigator.of(context).pop(true); },
+              onPressed: () { Navigator.of(context)!.pop(true); },
             ),
             TextButton(
               child: const Text('NO'),
-              onPressed: () { Navigator.of(context).pop(false); },
+              onPressed: () { Navigator.of(context)!.pop(false); },
             ),
           ],
         );
       },
-    ) ?? false;
+    ) as Future<bool>);
   }
 
   @override
@@ -195,7 +195,7 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
                       hintText: 'What do people call you?',
                       labelText: 'Name *',
                     ),
-                    onSaved: (String value) { person.name = value; },
+                    onSaved: (String? value) { person.name = value; },
                     validator: _validateName,
                   ),
                   const SizedBox(height: 24.0),
@@ -209,7 +209,7 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
                       prefixText: '+1',
                     ),
                     keyboardType: TextInputType.phone,
-                    onSaved: (String value) { person.phoneNumber = value; },
+                    onSaved: (String? value) { person.phoneNumber = value; },
                     validator: _validatePhoneNumber,
                     // TextInputFormatters are applied in sequence.
                     inputFormatters: <TextInputFormatter> [
@@ -228,7 +228,7 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
                       labelText: 'E-mail',
                     ),
                     keyboardType: TextInputType.emailAddress,
-                    onSaved: (String value) { person.email = value; },
+                    onSaved: (String? value) { person.email = value; },
                   ),
                   const SizedBox(height: 24.0),
                   TextFormField(
@@ -265,7 +265,7 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
                   ),
                   const SizedBox(height: 24.0),
                   TextFormField(
-                    enabled: person.password != null && person.password.isNotEmpty,
+                    enabled: person.password.isNotEmpty,
                     decoration: const InputDecoration(
                       border: UnderlineInputBorder(),
                       filled: true,

dev/integration_tests/flutter_gallery/lib/demo/shrine/app.dart

@@ -20,7 +20,7 @@ class ShrineApp extends StatefulWidget {
 class _ShrineAppState extends State<ShrineApp> with SingleTickerProviderStateMixin {
   // Controller to coordinate both the opening/closing of backdrop and sliding
   // of expanding bottom sheet
-  AnimationController _controller;
+  late AnimationController _controller;
 
   @override
   void initState() {
@@ -55,7 +55,7 @@ class _ShrineAppState extends State<ShrineApp> with SingleTickerProviderStateMix
   }
 }
 
-Route<dynamic> _getRoute(RouteSettings settings) {
+Route<dynamic>? _getRoute(RouteSettings settings) {
   if (settings.name != '/login') {
     return null;
   }
@@ -98,11 +98,11 @@ ThemeData _buildShrineTheme() {
 
 TextTheme _buildShrineTextTheme(TextTheme base) {
   return base.copyWith(
-    headline5: base.headline5.copyWith(fontWeight: FontWeight.w500),
-    headline6: base.headline6.copyWith(fontSize: 18.0),
-    caption: base.caption.copyWith(fontWeight: FontWeight.w400, fontSize: 14.0),
-    bodyText1: base.bodyText1.copyWith(fontWeight: FontWeight.w500, fontSize: 16.0),
-    button: base.button.copyWith(fontWeight: FontWeight.w500, fontSize: 14.0),
+    headline5: base.headline5!.copyWith(fontWeight: FontWeight.w500),
+    headline6: base.headline6!.copyWith(fontSize: 18.0),
+    caption: base.caption!.copyWith(fontWeight: FontWeight.w400, fontSize: 14.0),
+    bodyText1: base.bodyText1!.copyWith(fontWeight: FontWeight.w500, fontSize: 16.0),
+    button: base.button!.copyWith(fontWeight: FontWeight.w500, fontSize: 14.0),
   ).apply(
     fontFamily: 'Raleway',
     displayColor: kShrineBrown900,

dev/integration_tests/flutter_gallery/lib/demo/shrine/backdrop.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'package:flutter/material.dart';
-import 'package:meta/meta.dart';
 
 import 'package:flutter_gallery/demo/shrine/login.dart';
 
@@ -15,37 +14,37 @@ const double _kPeakVelocityProgress = 0.379146;
 class _TappableWhileStatusIs extends StatefulWidget {
   const _TappableWhileStatusIs(
       this.status, {
-        Key key,
+        Key? key,
         this.controller,
         this.child,
       }) : super(key: key);
 
-  final AnimationController controller;
+  final AnimationController? controller;
   final AnimationStatus status;
-  final Widget child;
+  final Widget? child;
 
   @override
   _TappableWhileStatusIsState createState() => _TappableWhileStatusIsState();
 }
 
 class _TappableWhileStatusIsState extends State<_TappableWhileStatusIs> {
-  bool _active;
+  bool? _active;
 
   @override
   void initState() {
     super.initState();
-    widget.controller.addStatusListener(_handleStatusChange);
-    _active = widget.controller.status == widget.status;
+    widget.controller!.addStatusListener(_handleStatusChange);
+    _active = widget.controller!.status == widget.status;
   }
 
   @override
   void dispose() {
-    widget.controller.removeStatusListener(_handleStatusChange);
+    widget.controller!.removeStatusListener(_handleStatusChange);
     super.dispose();
   }
 
   void _handleStatusChange(AnimationStatus status) {
-    final bool value = widget.controller.status == widget.status;
+    final bool value = widget.controller!.status == widget.status;
     if (_active != value) {
       setState(() {
         _active = value;
@@ -56,11 +55,11 @@ class _TappableWhileStatusIsState extends State<_TappableWhileStatusIs> {
   @override
   Widget build(BuildContext context) {
     Widget child = AbsorbPointer(
-      absorbing: !_active,
+      absorbing: !_active!,
       child: widget.child,
     );
 
-    if (!_active) {
+    if (!_active!) {
       child = FocusScope(
         canRequestFocus: false,
         debugLabel: '$_TappableWhileStatusIs',
@@ -73,13 +72,13 @@ class _TappableWhileStatusIsState extends State<_TappableWhileStatusIs> {
 
 class _FrontLayer extends StatelessWidget {
   const _FrontLayer({
-    Key key,
+    Key? key,
     this.onTap,
     this.child,
   }) : super(key: key);
 
-  final VoidCallback onTap;
-  final Widget child;
+  final VoidCallback? onTap;
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {
@@ -100,7 +99,7 @@ class _FrontLayer extends StatelessWidget {
             ),
           ),
           Expanded(
-            child: child,
+            child: child!,
           ),
         ],
       ),
@@ -110,16 +109,14 @@ class _FrontLayer extends StatelessWidget {
 
 class _BackdropTitle extends AnimatedWidget {
   const _BackdropTitle({
-    Key key,
-    Animation<double> listenable,
+    Key? key,
+    required Animation<double> listenable,
     this.onPress,
-    @required this.frontTitle,
-    @required this.backTitle,
-  }) : assert(frontTitle != null),
-       assert(backTitle != null),
-       super(key: key, listenable: listenable);
+    required this.frontTitle,
+    required this.backTitle,
+  }) : super(key: key, listenable: listenable);
 
-  final void Function() onPress;
+  final void Function()? onPress;
   final Widget frontTitle;
   final Widget backTitle;
 
@@ -131,7 +128,7 @@ class _BackdropTitle extends AnimatedWidget {
     );
 
     return DefaultTextStyle(
-      style: Theme.of(context).primaryTextTheme.headline6,
+      style: Theme.of(context).primaryTextTheme.headline6!,
       softWrap: false,
       overflow: TextOverflow.ellipsis,
       child: Row(children: <Widget>[
@@ -201,16 +198,12 @@ class _BackdropTitle extends AnimatedWidget {
 /// front or back layer is showing.
 class Backdrop extends StatefulWidget {
   const Backdrop({
-    @required this.frontLayer,
-    @required this.backLayer,
-    @required this.frontTitle,
-    @required this.backTitle,
-    @required this.controller,
-  }) : assert(frontLayer != null),
-       assert(backLayer != null),
-       assert(frontTitle != null),
-       assert(backTitle != null),
-       assert(controller != null);
+    required this.frontLayer,
+    required this.backLayer,
+    required this.frontTitle,
+    required this.backTitle,
+    required this.controller,
+  });
 
   final Widget frontLayer;
   final Widget backLayer;
@@ -224,8 +217,8 @@ class Backdrop extends StatefulWidget {
 
 class _BackdropState extends State<Backdrop> with SingleTickerProviderStateMixin {
   final GlobalKey _backdropKey = GlobalKey(debugLabel: 'Backdrop');
-  AnimationController _controller;
-  Animation<RelativeRect> _layerAnimation;
+  AnimationController? _controller;
+  late Animation<RelativeRect> _layerAnimation;
 
   @override
   void initState() {
@@ -235,19 +228,19 @@ class _BackdropState extends State<Backdrop> with SingleTickerProviderStateMixin
 
   @override
   void dispose() {
-    _controller.dispose();
+    _controller!.dispose();
     super.dispose();
   }
 
   bool get _frontLayerVisible {
-    final AnimationStatus status = _controller.status;
+    final AnimationStatus status = _controller!.status;
     return status == AnimationStatus.completed || status == AnimationStatus.forward;
   }
 
   void _toggleBackdropLayerVisibility() {
     // Call setState here to update layerAnimation if that's necessary
     setState(() {
-      _frontLayerVisible ? _controller.reverse() : _controller.forward();
+      _frontLayerVisible ? _controller!.reverse() : _controller!.forward();
     });
   }
 
@@ -267,7 +260,7 @@ class _BackdropState extends State<Backdrop> with SingleTickerProviderStateMixin
       firstWeight = _kPeakVelocityTime;
       secondWeight = 1.0 - _kPeakVelocityTime;
       animation = CurvedAnimation(
-        parent: _controller.view,
+        parent: _controller!.view,
         curve: const Interval(0.0, 0.78),
       );
     } else {
@@ -276,7 +269,7 @@ class _BackdropState extends State<Backdrop> with SingleTickerProviderStateMixin
       secondCurve = _kAccelerateCurve.flipped;
       firstWeight = 1.0 - _kPeakVelocityTime;
       secondWeight = _kPeakVelocityTime;
-      animation = _controller.view;
+      animation = _controller!.view;
     }
 
     return TweenSequence<RelativeRect>(
@@ -351,7 +344,7 @@ class _BackdropState extends State<Backdrop> with SingleTickerProviderStateMixin
       elevation: 0.0,
       titleSpacing: 0.0,
       title: _BackdropTitle(
-        listenable: _controller.view,
+        listenable: _controller!.view,
         onPress: _toggleBackdropLayerVisibility,
         frontTitle: widget.frontTitle,
         backTitle: widget.backTitle,

dev/integration_tests/flutter_gallery/lib/demo/shrine/category_menu_page.dart

@@ -11,22 +11,22 @@ import 'package:flutter_gallery/demo/shrine/model/product.dart';
 
 class CategoryMenuPage extends StatelessWidget {
   const CategoryMenuPage({
-    Key key,
+    Key? key,
     this.onCategoryTap,
   }) : super(key: key);
 
-  final VoidCallback onCategoryTap;
+  final VoidCallback? onCategoryTap;
 
   Widget _buildCategory(Category category, BuildContext context) {
     final String categoryString = category.toString().replaceAll('Category.', '').toUpperCase();
     final ThemeData theme = Theme.of(context);
     return ScopedModelDescendant<AppStateModel>(
-      builder: (BuildContext context, Widget child, AppStateModel model) =>
+      builder: (BuildContext context, Widget? child, AppStateModel model) =>
           GestureDetector(
             onTap: () {
               model.setCategory(category);
               if (onCategoryTap != null) {
-                onCategoryTap();
+                onCategoryTap!();
               }
             },
             child: model.selectedCategory == category
@@ -50,7 +50,7 @@ class CategoryMenuPage extends StatelessWidget {
                   padding: const EdgeInsets.symmetric(vertical: 16.0),
                   child: Text(
                     categoryString,
-                    style: theme.textTheme.bodyText1.copyWith(
+                    style: theme.textTheme.bodyText1!.copyWith(
                       color: kShrineBrown900.withAlpha(153)
                     ),
                     textAlign: TextAlign.center,

dev/integration_tests/flutter_gallery/lib/demo/shrine/expanding_bottom_sheet.dart

@@ -4,7 +4,6 @@
 
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
-import 'package:meta/meta.dart';
 import 'package:scoped_model/scoped_model.dart';
 
 import 'package:flutter_gallery/demo/shrine/colors.dart';
@@ -26,19 +25,16 @@ const double _kCornerRadius = 24.0;
 const double _kWidthForCartIcon = 64.0;
 
 class ExpandingBottomSheet extends StatefulWidget {
-  const ExpandingBottomSheet({Key key, @required this.hideController})
-      : assert(hideController != null),
-        super(key: key);
+  const ExpandingBottomSheet({Key? key, required this.hideController})
+      : super(key: key);
 
   final AnimationController hideController;
 
   @override
   _ExpandingBottomSheetState createState() => _ExpandingBottomSheetState();
 
-  static _ExpandingBottomSheetState of(BuildContext context, {bool isNullOk = false}) {
-    assert(isNullOk != null);
-    assert(context != null);
-    final _ExpandingBottomSheetState result = context.findAncestorStateOfType<_ExpandingBottomSheetState>();
+  static _ExpandingBottomSheetState? of(BuildContext context, {bool isNullOk = false}) {
+    final _ExpandingBottomSheetState? result = context.findAncestorStateOfType<_ExpandingBottomSheetState>();
     if (isNullOk || result != null) {
       return result;
     }
@@ -53,11 +49,11 @@ class ExpandingBottomSheet extends StatefulWidget {
 // curve formula. It's quintic, not cubic. But it _can_ be expressed as one
 // curve followed by another, which we do here.
 Animation<T> _getEmphasizedEasingAnimation<T>({
-  @required T begin,
-  @required T peak,
-  @required T end,
-  @required bool isForward,
-  @required Animation<double> parent,
+  required T begin,
+  required T peak,
+  required T end,
+  required bool isForward,
+  required Animation<double> parent,
 }) {
   Curve firstCurve;
   Curve secondCurve;
@@ -98,7 +94,7 @@ Animation<T> _getEmphasizedEasingAnimation<T>({
 
 // Calculates the value where two double Animations should be joined. Used by
 // callers of _getEmphasisedEasing<double>().
-double _getPeakPoint({double begin, double end}) {
+double _getPeakPoint({required double begin, required double end}) {
   return begin + (end - begin) * _kPeakVelocityProgress;
 }
 
@@ -111,15 +107,15 @@ class _ExpandingBottomSheetState extends State<ExpandingBottomSheet> with Ticker
   double _width = _kWidthForCartIcon;
 
   // Controller for the opening and closing of the ExpandingBottomSheet
-  AnimationController _controller;
+  late AnimationController _controller;
 
   // Animations for the opening and closing of the ExpandingBottomSheet
-  Animation<double> _widthAnimation;
-  Animation<double> _heightAnimation;
-  Animation<double> _thumbnailOpacityAnimation;
-  Animation<double> _cartOpacityAnimation;
-  Animation<double> _shapeAnimation;
-  Animation<Offset> _slideAnimation;
+  late Animation<double> _widthAnimation;
+  late Animation<double> _heightAnimation;
+  late Animation<double> _thumbnailOpacityAnimation;
+  late Animation<double> _cartOpacityAnimation;
+  late Animation<double> _shapeAnimation;
+  late Animation<Offset> _slideAnimation;
 
   @override
   void initState() {
@@ -308,7 +304,7 @@ class _ExpandingBottomSheetState extends State<ExpandingBottomSheet> with Ticker
     );
   }
 
-  Widget _buildCart(BuildContext context, Widget child) {
+  Widget _buildCart(BuildContext context, Widget? child) {
     // numProducts is the number of different products in the cart (does not
     // include multiples of the same product).
     final AppStateModel model = ScopedModel.of<AppStateModel>(context);
@@ -349,7 +345,7 @@ class _ExpandingBottomSheetState extends State<ExpandingBottomSheet> with Ticker
   }
 
   // Builder for the hide and reveal animation when the backdrop opens and closes
-  Widget _buildSlideAnimation(BuildContext context, Widget child) {
+  Widget _buildSlideAnimation(BuildContext context, Widget? child) {
     _slideAnimation = _getEmphasizedEasingAnimation(
       begin: const Offset(1.0, 0.0),
       peak: const Offset(_kPeakVelocityProgress, 0.0),
@@ -393,7 +389,7 @@ class _ExpandingBottomSheetState extends State<ExpandingBottomSheet> with Ticker
             behavior: HitTestBehavior.opaque,
             onTap: open,
             child: ScopedModelDescendant<AppStateModel>(
-              builder: (BuildContext context, Widget child, AppStateModel model) {
+              builder: (BuildContext context, Widget? child, AppStateModel model) {
                 return AnimatedBuilder(
                   builder: _buildCart,
                   animation: _controller,
@@ -417,10 +413,10 @@ class _ProductThumbnailRowState extends State<ProductThumbnailRow> {
 
   // _list represents what's currently on screen. If _internalList updates,
   // it will need to be updated to match it.
-  _ListModel _list;
+  late _ListModel _list;
 
   // _internalList represents the list as it is updated by the AppStateModel.
-  List<int> _internalList;
+  late List<int> _internalList;
 
   @override
   void initState() {
@@ -436,7 +432,6 @@ class _ProductThumbnailRowState extends State<ProductThumbnailRow> {
   Product _productWithId(int productId) {
     final AppStateModel model = ScopedModel.of<AppStateModel>(context);
     final Product product = model.getProductById(productId);
-    assert(product != null);
     return product;
   }
 
@@ -510,7 +505,7 @@ class _ProductThumbnailRowState extends State<ProductThumbnailRow> {
   Widget build(BuildContext context) {
     _updateLists();
     return ScopedModelDescendant<AppStateModel>(
-      builder: (BuildContext context, Widget child, AppStateModel model) => _buildAnimatedList(),
+      builder: (BuildContext context, Widget? child, AppStateModel model) => _buildAnimatedList(),
     );
   }
 }
@@ -529,7 +524,7 @@ class ExtraProductsNumber extends StatelessWidget {
     final int numProducts = products.length;
     if (numProducts > 3) {
       for (int i = 3; i < numProducts; i++) {
-        overflow += productMap[products[i]];
+        overflow += productMap[products[i]]!;
       }
     }
     return overflow;
@@ -553,7 +548,7 @@ class ExtraProductsNumber extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return ScopedModelDescendant<AppStateModel>(
-      builder: (BuildContext builder, Widget child, AppStateModel model) => _buildOverflow(model, context),
+      builder: (BuildContext builder, Widget? child, AppStateModel model) => _buildOverflow(model, context),
     );
   }
 }
@@ -594,18 +589,16 @@ class ProductThumbnail extends StatelessWidget {
 // _ListModel manipulates an internal list and an AnimatedList
 class _ListModel {
   _ListModel({
-    @required this.listKey,
-    @required this.removedItemBuilder,
-    Iterable<int> initialItems,
-  }) : assert(listKey != null),
-       assert(removedItemBuilder != null),
-       _items = initialItems?.toList() ?? <int>[];
+    required this.listKey,
+    required this.removedItemBuilder,
+    Iterable<int>? initialItems,
+  }) : _items = initialItems?.toList() ?? <int>[];
 
   final GlobalKey<AnimatedListState> listKey;
   final Widget Function(int item, BuildContext context, Animation<double> animation) removedItemBuilder;
   final List<int> _items;
 
-  AnimatedListState get _animatedList => listKey.currentState;
+  AnimatedListState? get _animatedList => listKey.currentState;
 
   void add(int product) {
     _insert(_items.length, product);
@@ -613,7 +606,7 @@ class _ListModel {
 
   void _insert(int index, int item) {
     _items.insert(index, item);
-    _animatedList.insertItem(index, duration: const Duration(milliseconds: 225));
+    _animatedList!.insertItem(index, duration: const Duration(milliseconds: 225));
   }
 
   void remove(int product) {
@@ -625,11 +618,9 @@ class _ListModel {
 
   void _removeAt(int index) {
     final int removedItem = _items.removeAt(index);
-    if (removedItem != null) {
-      _animatedList.removeItem(index, (BuildContext context, Animation<double> animation) {
-        return removedItemBuilder(removedItem, context, animation);
-      });
-    }
+    _animatedList!.removeItem(index, (BuildContext context, Animation<double> animation) {
+      return removedItemBuilder(removedItem, context, animation);
+    });
   }
 
   int get length => _items.length;

dev/integration_tests/flutter_gallery/lib/demo/shrine/home.dart

@@ -19,7 +19,7 @@ class ProductPage extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return ScopedModelDescendant<AppStateModel>(
-      builder: (BuildContext context, Widget child, AppStateModel model) {
+      builder: (BuildContext context, Widget? child, AppStateModel model) {
         return AsymmetricView(products: model.getProducts());
       });
   }
@@ -29,17 +29,18 @@ class HomePage extends StatelessWidget {
   const HomePage({
     this.expandingBottomSheet,
     this.backdrop,
-    Key key,
+    Key? key,
   }) : super(key: key);
 
-  final ExpandingBottomSheet expandingBottomSheet;
-  final Backdrop backdrop;
+  final ExpandingBottomSheet? expandingBottomSheet;
+  final Backdrop? backdrop;
 
   @override
   Widget build(BuildContext context) {
     return Stack(
       children: <Widget>[
-        backdrop,
+        if (backdrop != null)
+          backdrop!,
         Align(child: expandingBottomSheet, alignment: Alignment.bottomRight),
       ],
     );

dev/integration_tests/flutter_gallery/lib/demo/shrine/login.dart

@@ -35,7 +35,7 @@ class _LoginPageState extends State<LoginPage> {
             // The login screen is immediately displayed on top of the Shrine
             // home screen using onGenerateRoute and so rootNavigator must be
             // set to true in order to get out of Shrine completely.
-            Navigator.of(context, rootNavigator: true).pop();
+            Navigator.of(context, rootNavigator: true)!.pop();
           },
         ),
       ),
@@ -95,7 +95,7 @@ class _LoginPageState extends State<LoginPage> {
                         // the Shrine home screen using onGenerateRoute and so
                         // rootNavigator must be set to true in order to get out
                         // of Shrine completely.
-                        Navigator.of(context, rootNavigator: true).pop();
+                        Navigator.of(context, rootNavigator: true)!.pop();
                       },
                       child: const Text('CANCEL'),
                     ),
@@ -123,15 +123,15 @@ class _LoginPageState extends State<LoginPage> {
 }
 
 class PrimaryColorOverride extends StatelessWidget {
-  const PrimaryColorOverride({Key key, this.color, this.child}) : super(key: key);
+  const PrimaryColorOverride({Key? key, this.color, this.child}) : super(key: key);
 
-  final Color color;
-  final Widget child;
+  final Color? color;
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {
     return Theme(
-      child: child,
+      child: child!,
       data: Theme.of(context).copyWith(primaryColor: color),
     );
   }

dev/integration_tests/flutter_gallery/lib/demo/shrine/model/app_state_model.dart

@@ -12,7 +12,7 @@ double _shippingCostPerItem = 7.0;
 
 class AppStateModel extends Model {
   // All the available products.
-  List<Product> _availableProducts;
+  List<Product>? _availableProducts;
 
   // The currently selected category of products.
   Category _selectedCategory = Category.all;
@@ -30,7 +30,7 @@ class AppStateModel extends Model {
   // Totaled prices of the items in the cart.
   double get subtotalCost {
     return _productsInCart.keys
-      .map((int id) => _availableProducts[id].price * _productsInCart[id])
+      .map((int id) => _availableProducts![id].price * _productsInCart[id]!)
       .fold(0.0, (double sum, int e) => sum + e);
   }
 
@@ -52,9 +52,9 @@ class AppStateModel extends Model {
     }
 
     if (_selectedCategory == Category.all) {
-      return List<Product>.from(_availableProducts);
+      return List<Product>.from(_availableProducts!);
     } else {
-      return _availableProducts
+      return _availableProducts!
         .where((Product p) => p.category == _selectedCategory)
         .toList();
     }
@@ -62,10 +62,11 @@ class AppStateModel extends Model {
 
   // Adds a product to the cart.
   void addProductToCart(int productId) {
-    if (!_productsInCart.containsKey(productId)) {
+    final int? value = _productsInCart[productId];
+    if (value == null) {
       _productsInCart[productId] = 1;
     } else {
-      _productsInCart[productId]++;
+      _productsInCart[productId] = value+1;
     }
 
     notifyListeners();
@@ -73,11 +74,13 @@ class AppStateModel extends Model {
 
   // Removes an item from the cart.
   void removeItemFromCart(int productId) {
-    if (_productsInCart.containsKey(productId)) {
+    final int? value = _productsInCart[productId];
+
+    if (value != null) {
       if (_productsInCart[productId] == 1) {
         _productsInCart.remove(productId);
       } else {
-        _productsInCart[productId]--;
+        _productsInCart[productId] = value - 1;
       }
     }
 
@@ -86,7 +89,7 @@ class AppStateModel extends Model {
 
   // Returns the Product instance matching the provided id.
   Product getProductById(int id) {
-    return _availableProducts.firstWhere((Product p) => p.id == id);
+    return _availableProducts!.firstWhere((Product p) => p.id == id);
   }
 
   // Removes everything from the cart.

dev/integration_tests/flutter_gallery/lib/demo/shrine/model/product.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/foundation.dart';
-
 enum Category {
   all,
   accessories,
@@ -13,16 +11,12 @@ enum Category {
 
 class Product {
   const Product({
-    @required this.category,
-    @required this.id,
-    @required this.isFeatured,
-    @required this.name,
-    @required this.price,
-  }) : assert(category != null),
-       assert(id != null),
-       assert(isFeatured != null),
-       assert(name != null),
-       assert(price != null);
+    required this.category,
+    required this.id,
+    required this.isFeatured,
+    required this.name,
+    required this.price,
+  });
 
   final Category category;
   final int id;

dev/integration_tests/flutter_gallery/lib/demo/shrine/supplemental/asymmetric_view.dart

@@ -8,12 +8,12 @@ import 'package:flutter_gallery/demo/shrine/model/product.dart';
 import 'package:flutter_gallery/demo/shrine/supplemental/product_columns.dart';
 
 class AsymmetricView extends StatelessWidget {
-  const AsymmetricView({Key key, this.products}) : super(key: key);
+  const AsymmetricView({Key? key, this.products}) : super(key: key);
 
-  final List<Product> products;
+  final List<Product>? products;
 
   List<Container> _buildColumns(BuildContext context) {
-    if (products == null || products.isEmpty) {
+    if (products == null || products!.isEmpty) {
       return const <Container>[];
     }
 
@@ -25,23 +25,23 @@ class AsymmetricView extends StatelessWidget {
     // some kinda awkward math so we use _evenCasesIndex and _oddCasesIndex as
     // helpers for creating the index of the product list that will correspond
     // to the index of the list of columns.
-    return List<Container>.generate(_listItemCount(products.length), (int index) {
+    return List<Container>.generate(_listItemCount(products!.length), (int index) {
       double width = .59 * MediaQuery.of(context).size.width;
       Widget column;
       if (index.isEven) {
         /// Even cases
         final int bottom = _evenCasesIndex(index);
         column = TwoProductCardColumn(
-          bottom: products[bottom],
-          top: products.length - 1 >= bottom + 1
-            ? products[bottom + 1]
+          bottom: products![bottom],
+          top: products!.length - 1 >= bottom + 1
+            ? products![bottom + 1]
             : null,
         );
         width += 32.0;
       } else {
         /// Odd cases
         column = OneProductCardColumn(
-          product: products[_oddCasesIndex(index)],
+          product: products![_oddCasesIndex(index)],
         );
       }
       return Container(

dev/integration_tests/flutter_gallery/lib/demo/shrine/supplemental/product_columns.dart

@@ -9,11 +9,11 @@ import 'package:flutter_gallery/demo/shrine/supplemental/product_card.dart';
 
 class TwoProductCardColumn extends StatelessWidget {
   const TwoProductCardColumn({
-    @required this.bottom,
+    required this.bottom,
     this.top,
-  }) : assert(bottom != null);
+  });
 
-  final Product bottom, top;
+  final Product? bottom, top;
 
   @override
   Widget build(BuildContext context) {
@@ -59,7 +59,7 @@ class TwoProductCardColumn extends StatelessWidget {
 class OneProductCardColumn extends StatelessWidget {
   const OneProductCardColumn({this.product});
 
-  final Product product;
+  final Product? product;
 
   @override
   Widget build(BuildContext context) {

dev/integration_tests/flutter_gallery/lib/demo/shrine_demo.dart

@@ -6,7 +6,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_gallery/demo/shrine/app.dart';
 
 class ShrineDemo extends StatelessWidget {
-  const ShrineDemo({ Key key }) : super(key: key);
+  const ShrineDemo({ Key? key }) : super(key: key);
 
   static const String routeName = '/shrine'; // Used by the Gallery app.
 

dev/integration_tests/flutter_gallery/lib/demo/transformations/transformations_demo_inertial_motion.dart

@@ -60,7 +60,7 @@ class InertialMotion {
 
   // Solve the equation of motion to find the position at a given point in time
   // in one dimension.
-  double _getPosition({double r0, double v0, int t, double a}) {
+  double _getPosition({required double r0, required double v0, required int t, required double a}) {
     // Stop movement when it would otherwise reverse direction.
     final double stopTime = (v0 / a).abs();
     if (t > stopTime) {