migrate some material files to nullsafty (#66858)

* migrate some material files to nullsafty

* address review comments

packages/flutter/lib/src/material/card_theme.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.
 
-// @dart = 2.8
-
 import 'dart:ui' show lerpDouble;
 
 import 'package:flutter/foundation.dart';
@@ -46,44 +44,44 @@ class CardTheme with Diagnosticable {
   /// Default value for [Card.clipBehavior].
   ///
   /// If null, [Card] uses [Clip.none].
-  final Clip clipBehavior;
+  final Clip? clipBehavior;
 
   /// Default value for [Card.color].
   ///
   /// If null, [Card] uses [ThemeData.cardColor].
-  final Color color;
+  final Color? color;
 
   /// Default value for [Card.shadowColor].
   ///
   /// If null, [Card] defaults to fully opaque black.
-  final Color shadowColor;
+  final Color? shadowColor;
 
   /// Default value for [Card.elevation].
   ///
   /// If null, [Card] uses a default of 1.0.
-  final double elevation;
+  final double? elevation;
 
   /// Default value for [Card.margin].
   ///
   /// If null, [Card] uses a default margin of 4.0 logical pixels on all sides:
   /// `EdgeInsets.all(4.0)`.
-  final EdgeInsetsGeometry margin;
+  final EdgeInsetsGeometry? margin;
 
   /// Default value for [Card.shape].
   ///
   /// If null, [Card] then uses a [RoundedRectangleBorder] with a circular
   /// corner radius of 4.0.
-  final ShapeBorder shape;
+  final ShapeBorder? shape;
 
   /// Creates a copy of this object with the given fields replaced with the
   /// new values.
   CardTheme copyWith({
-    Clip clipBehavior,
-    Color color,
-    Color shadowColor,
-    double elevation,
-    EdgeInsetsGeometry margin,
-    ShapeBorder shape,
+    Clip? clipBehavior,
+    Color? color,
+    Color? shadowColor,
+    double? elevation,
+    EdgeInsetsGeometry? margin,
+    ShapeBorder? shape,
   }) {
     return CardTheme(
       clipBehavior: clipBehavior ?? this.clipBehavior,
@@ -97,7 +95,7 @@ class CardTheme with Diagnosticable {
 
   /// The [ThemeData.cardTheme] property of the ambient [Theme].
   static CardTheme of(BuildContext context) {
-    return Theme.of(context).cardTheme;
+    return Theme.of(context)!.cardTheme;
   }
 
   /// Linearly interpolate between two Card themes.
@@ -105,7 +103,7 @@ class CardTheme with Diagnosticable {
   /// The argument `t` must not be null.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static CardTheme lerp(CardTheme a, CardTheme b, double t) {
+  static CardTheme lerp(CardTheme? a, CardTheme? b, double t) {
     assert(t != null);
     return CardTheme(
       clipBehavior: t < 0.5 ? a?.clipBehavior : b?.clipBehavior,

packages/flutter/lib/src/material/debug.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.
 
-// @dart = 2.8
-
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 

packages/flutter/lib/src/material/elevation_overlay.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.
 
-// @dart = 2.8
-
 import 'dart:math' as math;
 
 import 'package:flutter/widgets.dart';
@@ -45,7 +43,7 @@ class ElevationOverlay {
   ///  * <https://material.io/design/color/dark-theme.html>, which specifies how
   ///    the overlay should be applied.
   static Color applyOverlay(BuildContext context, Color color, double elevation) {
-    final ThemeData theme = Theme.of(context);
+    final ThemeData theme = Theme.of(context)!;
     if (elevation > 0.0 &&
         theme.applyElevationOverlayColor &&
         theme.brightness == Brightness.dark &&
@@ -63,7 +61,7 @@ class ElevationOverlay {
   ///  * https://material.io/design/color/dark-theme.html#properties which
   ///    specifies the exact overlay values for a given elevation.
   static Color overlayColor(BuildContext context, double elevation) {
-    final ThemeData theme = Theme.of(context);
+    final ThemeData theme = Theme.of(context)!;
     // Compute the opacity for the given elevation
     // This formula matches the values in the spec:
     // https://material.io/design/color/dark-theme.html#properties

packages/flutter/lib/src/material/feedback.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.
 
-// @dart = 2.8
-
 import 'package:flutter/rendering.dart';
 import 'package:flutter/semantics.dart';
 import 'package:flutter/services.dart';
@@ -94,7 +92,7 @@ class Feedback {
   ///  * [wrapForTap] to trigger platform-specific feedback before executing a
   ///    [GestureTapCallback].
   static Future<void> forTap(BuildContext context) async {
-    context.findRenderObject().sendSemanticsEvent(const TapSemanticEvent());
+    context.findRenderObject()!.sendSemanticsEvent(const TapSemanticEvent());
     switch (_platform(context)) {
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
@@ -104,10 +102,7 @@ class Feedback {
       case TargetPlatform.macOS:
       case TargetPlatform.windows:
         return Future<void>.value();
-        break;
     }
-    assert(false, 'Unhandled TargetPlatform ${_platform(context)}');
-    return Future<void>.value();
   }
 
   /// Wraps a [GestureTapCallback] to provide platform specific feedback for a
@@ -120,7 +115,7 @@ class Feedback {
   ///
   ///  * [forTap] to just trigger the platform-specific feedback without wrapping
   ///    a [GestureTapCallback].
-  static GestureTapCallback wrapForTap(GestureTapCallback callback, BuildContext context) {
+  static GestureTapCallback? wrapForTap(GestureTapCallback? callback, BuildContext context) {
     if (callback == null)
       return null;
     return () {
@@ -139,7 +134,7 @@ class Feedback {
   ///  * [wrapForLongPress] to trigger platform-specific feedback before
   ///    executing a [GestureLongPressCallback].
   static Future<void> forLongPress(BuildContext context) {
-    context.findRenderObject().sendSemanticsEvent(const LongPressSemanticsEvent());
+    context.findRenderObject()!.sendSemanticsEvent(const LongPressSemanticsEvent());
     switch (_platform(context)) {
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
@@ -149,10 +144,7 @@ class Feedback {
       case TargetPlatform.macOS:
       case TargetPlatform.windows:
         return Future<void>.value();
-        break;
     }
-    assert(false, 'Unhandled TargetPlatform ${_platform(context)}');
-    return Future<void>.value();
   }
 
   /// Wraps a [GestureLongPressCallback] to provide platform specific feedback
@@ -166,7 +158,7 @@ class Feedback {
   ///
   ///  * [forLongPress] to just trigger the platform-specific feedback without
   ///    wrapping a [GestureLongPressCallback].
-  static GestureLongPressCallback wrapForLongPress(GestureLongPressCallback callback, BuildContext context) {
+  static GestureLongPressCallback? wrapForLongPress(GestureLongPressCallback? callback, BuildContext context) {
     if (callback == null)
       return null;
     return () {
@@ -175,5 +167,5 @@ class Feedback {
     };
   }
 
-  static TargetPlatform _platform(BuildContext context) => Theme.of(context).platform;
+  static TargetPlatform _platform(BuildContext context) => Theme.of(context)!.platform;
 }

packages/flutter/lib/src/material/material.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.
 
-// @dart = 2.8
-
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -56,7 +54,7 @@ enum MaterialType {
 ///
 ///  * [MaterialType]
 ///  * [Material]
-final Map<MaterialType, BorderRadius> kMaterialEdges = <MaterialType, BorderRadius>{
+final Map<MaterialType, BorderRadius?> kMaterialEdges = <MaterialType, BorderRadius?>{
   MaterialType.canvas: null,
   MaterialType.card: BorderRadius.circular(2.0),
   MaterialType.circle: null,
@@ -69,7 +67,7 @@ final Map<MaterialType, BorderRadius> kMaterialEdges = <MaterialType, BorderRadi
 /// Typically obtained via [Material.of].
 abstract class MaterialInkController {
   /// The color of the material.
-  Color get color;
+  Color? get color;
 
   /// The ticker provider used by the controller.
   ///
@@ -177,7 +175,7 @@ class Material extends StatefulWidget {
   /// [MaterialType.circle]. In both cases, these restrictions are intended to
   /// catch likely errors.
   const Material({
-    Key key,
+    Key? key,
     this.type = MaterialType.canvas,
     this.elevation = 0.0,
     this.color,
@@ -201,7 +199,7 @@ class Material extends StatefulWidget {
   /// The widget below this widget in the tree.
   ///
   /// {@macro flutter.widgets.child}
-  final Widget child;
+  final Widget? child;
 
   /// The kind of material to show (e.g., card or canvas). This
   /// affects the shape of the widget, the roundness of its corners if
@@ -242,7 +240,7 @@ class Material extends StatefulWidget {
   /// composited on top of this color to indicate the elevation.
   ///
   /// By default, the color is derived from the [type] of material.
-  final Color color;
+  final Color? color;
 
   /// The color to paint the shadow below the material.
   ///
@@ -257,10 +255,10 @@ class Material extends StatefulWidget {
   ///
   ///  * [ThemeData.applyElevationOverlayColor], which turns elevation overlay
   /// on or off for dark themes.
-  final Color shadowColor;
+  final Color? shadowColor;
 
   /// The typographical style to use for text within this material.
-  final TextStyle textStyle;
+  final TextStyle? textStyle;
 
   /// Defines the material's shape as well its shadow.
   ///
@@ -269,7 +267,7 @@ class Material extends StatefulWidget {
   ///
   /// A shadow is only displayed if the [elevation] is greater than
   /// zero.
-  final ShapeBorder shape;
+  final ShapeBorder? shape;
 
   /// Whether to paint the [shape] border in front of the [child].
   ///
@@ -302,7 +300,7 @@ class Material extends StatefulWidget {
   /// If [shape] is non null then the border radius is ignored.
   ///
   /// Must be null if [type] is [MaterialType.circle].
-  final BorderRadiusGeometry borderRadius;
+  final BorderRadiusGeometry? borderRadius;
 
   /// The ink controller from the closest instance of this class that
   /// encloses the given context.
@@ -312,9 +310,8 @@ class Material extends StatefulWidget {
   /// ```dart
   /// MaterialInkController inkController = Material.of(context);
   /// ```
-  static MaterialInkController of(BuildContext context) {
-    final _RenderInkFeatures result = context.findAncestorRenderObjectOfType<_RenderInkFeatures>();
-    return result;
+  static MaterialInkController? of(BuildContext context) {
+    return context.findAncestorRenderObjectOfType<_RenderInkFeatures>();
   }
 
   @override
@@ -340,9 +337,9 @@ class Material extends StatefulWidget {
 class _MaterialState extends State<Material> with TickerProviderStateMixin {
   final GlobalKey _inkFeatureRenderer = GlobalKey(debugLabel: 'ink renderer');
 
-  Color _getBackgroundColor(BuildContext context) {
-    final ThemeData theme = Theme.of(context);
-    Color color = widget.color;
+  Color? _getBackgroundColor(BuildContext context) {
+    final ThemeData theme = Theme.of(context)!;
+    Color? color = widget.color;
     if (color == null) {
       switch (widget.type) {
         case MaterialType.canvas:
@@ -360,7 +357,7 @@ class _MaterialState extends State<Material> with TickerProviderStateMixin {
 
   @override
   Widget build(BuildContext context) {
-    final Color backgroundColor = _getBackgroundColor(context);
+    final Color? backgroundColor = _getBackgroundColor(context);
     assert(
       backgroundColor != null || widget.type == MaterialType.transparency,
       'If Material type is not MaterialType.transparency, a color must '
@@ -368,17 +365,17 @@ class _MaterialState extends State<Material> with TickerProviderStateMixin {
       'in the theme (ex. canvasColor != null if type is set to '
       'MaterialType.canvas)'
     );
-    Widget contents = widget.child;
+    Widget? contents = widget.child;
     if (contents != null) {
       contents = AnimatedDefaultTextStyle(
-        style: widget.textStyle ?? Theme.of(context).textTheme.bodyText2,
+        style: widget.textStyle ?? Theme.of(context)!.textTheme.bodyText2!,
         duration: widget.animationDuration,
         child: contents,
       );
     }
     contents = NotificationListener<LayoutChangedNotification>(
       onNotification: (LayoutChangedNotification notification) {
-        final _RenderInkFeatures renderer = _inkFeatureRenderer.currentContext.findRenderObject() as _RenderInkFeatures;
+        final _RenderInkFeatures renderer = _inkFeatureRenderer.currentContext!.findRenderObject()! as _RenderInkFeatures;
         renderer._didChangeLayout();
         return false;
       },
@@ -408,8 +405,8 @@ class _MaterialState extends State<Material> with TickerProviderStateMixin {
         clipBehavior: widget.clipBehavior,
         borderRadius: BorderRadius.zero,
         elevation: widget.elevation,
-        color: ElevationOverlay.applyOverlay(context, backgroundColor, widget.elevation),
-        shadowColor: widget.shadowColor ?? Theme.of(context).shadowColor,
+        color: ElevationOverlay.applyOverlay(context, backgroundColor!, widget.elevation),
+        shadowColor: widget.shadowColor ?? Theme.of(context)!.shadowColor,
         animateColor: false,
         child: contents,
       );
@@ -433,17 +430,17 @@ class _MaterialState extends State<Material> with TickerProviderStateMixin {
       borderOnForeground: widget.borderOnForeground,
       clipBehavior: widget.clipBehavior,
       elevation: widget.elevation,
-      color: backgroundColor,
-      shadowColor: widget.shadowColor ?? Theme.of(context).shadowColor,
+      color: backgroundColor!,
+      shadowColor: widget.shadowColor ?? Theme.of(context)!.shadowColor,
       child: contents,
     );
   }
 
   static Widget _transparentInterior({
-    @required BuildContext context,
-    @required ShapeBorder shape,
-    @required Clip clipBehavior,
-    @required Widget contents,
+    required BuildContext context,
+    required ShapeBorder shape,
+    required Clip clipBehavior,
+    required Widget contents,
   }) {
     final _ShapeBorderPaint child = _ShapeBorderPaint(
       child: contents,
@@ -471,9 +468,9 @@ class _MaterialState extends State<Material> with TickerProviderStateMixin {
   // Material class documentation.
   ShapeBorder _getShape() {
     if (widget.shape != null)
-      return widget.shape;
+      return widget.shape!;
     if (widget.borderRadius != null)
-      return RoundedRectangleBorder(borderRadius: widget.borderRadius);
+      return RoundedRectangleBorder(borderRadius: widget.borderRadius!);
     switch (widget.type) {
       case MaterialType.canvas:
       case MaterialType.transparency:
@@ -482,21 +479,20 @@ class _MaterialState extends State<Material> with TickerProviderStateMixin {
       case MaterialType.card:
       case MaterialType.button:
         return RoundedRectangleBorder(
-          borderRadius: widget.borderRadius ?? kMaterialEdges[widget.type],
+          borderRadius: widget.borderRadius ?? kMaterialEdges[widget.type]!,
         );
 
       case MaterialType.circle:
         return const CircleBorder();
     }
-    return const RoundedRectangleBorder();
   }
 }
 
 class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController {
   _RenderInkFeatures({
-    RenderBox child,
-    @required this.vsync,
-    this.absorbHitTest,
+    RenderBox? child,
+    required this.vsync,
+    required this.absorbHitTest,
     this.color,
   }) : assert(vsync != null),
        super(child);
@@ -511,30 +507,30 @@ class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController
   // The actual painting of this color is done by a Container in the
   // MaterialState build method.
   @override
-  Color color;
+  Color? color;
 
   bool absorbHitTest;
 
-  List<InkFeature> _inkFeatures;
+  List<InkFeature>? _inkFeatures;
 
   @override
   void addInkFeature(InkFeature feature) {
     assert(!feature._debugDisposed);
     assert(feature._controller == this);
     _inkFeatures ??= <InkFeature>[];
-    assert(!_inkFeatures.contains(feature));
-    _inkFeatures.add(feature);
+    assert(!_inkFeatures!.contains(feature));
+    _inkFeatures!.add(feature);
     markNeedsPaint();
   }
 
   void _removeFeature(InkFeature feature) {
     assert(_inkFeatures != null);
-    _inkFeatures.remove(feature);
+    _inkFeatures!.remove(feature);
     markNeedsPaint();
   }
 
   void _didChangeLayout() {
-    if (_inkFeatures != null && _inkFeatures.isNotEmpty)
+    if (_inkFeatures != null && _inkFeatures!.isNotEmpty)
       markNeedsPaint();
   }
 
@@ -543,12 +539,12 @@ class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (_inkFeatures != null && _inkFeatures.isNotEmpty) {
+    if (_inkFeatures != null && _inkFeatures!.isNotEmpty) {
       final Canvas canvas = context.canvas;
       canvas.save();
       canvas.translate(offset.dx, offset.dy);
       canvas.clipRect(Offset.zero & size);
-      for (final InkFeature inkFeature in _inkFeatures)
+      for (final InkFeature inkFeature in _inkFeatures!)
         inkFeature._paint(canvas);
       canvas.restore();
     }
@@ -558,17 +554,17 @@ class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController
 
 class _InkFeatures extends SingleChildRenderObjectWidget {
   const _InkFeatures({
-    Key key,
+    Key? key,
     this.color,
-    @required this.vsync,
-    @required this.absorbHitTest,
-    Widget child,
+    required this.vsync,
+    required this.absorbHitTest,
+    Widget? child,
   }) : super(key: key, child: child);
 
   // This widget must be owned by a MaterialState, which must be provided as the vsync.
   // This relationship must be 1:1 and cannot change for the lifetime of the MaterialState.
 
-  final Color color;
+  final Color? color;
 
   final TickerProvider vsync;
 
@@ -599,8 +595,8 @@ class _InkFeatures extends SingleChildRenderObjectWidget {
 abstract class InkFeature {
   /// Initializes fields for subclasses.
   InkFeature({
-    @required MaterialInkController controller,
-    @required this.referenceBox,
+    required MaterialInkController controller,
+    required this.referenceBox,
     this.onRemoved,
   }) : assert(controller != null),
        assert(referenceBox != null),
@@ -617,7 +613,7 @@ abstract class InkFeature {
   final RenderBox referenceBox;
 
   /// Called when the ink feature is no longer visible on the material.
-  final VoidCallback onRemoved;
+  final VoidCallback? onRemoved;
 
   bool _debugDisposed = false;
 
@@ -631,7 +627,7 @@ abstract class InkFeature {
     }());
     _controller._removeFeature(this);
     if (onRemoved != null)
-      onRemoved();
+      onRemoved!();
   }
 
   void _paint(Canvas canvas) {
@@ -667,16 +663,16 @@ abstract class InkFeature {
 /// An interpolation between two [ShapeBorder]s.
 ///
 /// This class specializes the interpolation of [Tween] to use [ShapeBorder.lerp].
-class ShapeBorderTween extends Tween<ShapeBorder> {
+class ShapeBorderTween extends Tween<ShapeBorder?> {
   /// Creates a [ShapeBorder] tween.
   ///
   /// the [begin] and [end] properties may be null; see [ShapeBorder.lerp] for
   /// the null handling semantics.
-  ShapeBorderTween({ShapeBorder begin, ShapeBorder end}) : super(begin: begin, end: end);
+  ShapeBorderTween({ShapeBorder? begin, ShapeBorder? end}) : super(begin: begin, end: end);
 
   /// Returns the value this tween has at the given animation clock value.
   @override
-  ShapeBorder lerp(double t) {
+  ShapeBorder? lerp(double t) {
     return ShapeBorder.lerp(begin, end, t);
   }
 }
@@ -691,16 +687,16 @@ class _MaterialInterior extends ImplicitlyAnimatedWidget {
   /// must not be null. The [elevation] must be specified and greater than or
   /// equal to zero.
   const _MaterialInterior({
-    Key key,
-    @required this.child,
-    @required this.shape,
+    Key? key,
+    required this.child,
+    required this.shape,
     this.borderOnForeground = true,
     this.clipBehavior = Clip.none,
-    @required this.elevation,
-    @required this.color,
-    @required this.shadowColor,
+    required this.elevation,
+    required this.color,
+    required this.shadowColor,
     Curve curve = Curves.linear,
-    @required Duration duration,
+    required Duration duration,
   }) : assert(child != null),
        assert(shape != null),
        assert(clipBehavior != null),
@@ -757,9 +753,9 @@ class _MaterialInterior extends ImplicitlyAnimatedWidget {
 }
 
 class _MaterialInteriorState extends AnimatedWidgetBaseState<_MaterialInterior> {
-  Tween<double> _elevation;
-  ColorTween _shadowColor;
-  ShapeBorderTween _border;
+  Tween<double>? _elevation;
+  ColorTween? _shadowColor;
+  ShapeBorderTween? _border;
 
   @override
   void forEachTween(TweenVisitor<dynamic> visitor) {
@@ -782,8 +778,8 @@ class _MaterialInteriorState extends AnimatedWidgetBaseState<_MaterialInterior>
 
   @override
   Widget build(BuildContext context) {
-    final ShapeBorder shape = _border.evaluate(animation);
-    final double elevation = _elevation.evaluate(animation);
+    final ShapeBorder shape = _border!.evaluate(animation!)!;
+    final double elevation = _elevation!.evaluate(animation!);
     return PhysicalShape(
       child: _ShapeBorderPaint(
         child: widget.child,
@@ -797,15 +793,15 @@ class _MaterialInteriorState extends AnimatedWidgetBaseState<_MaterialInterior>
       clipBehavior: widget.clipBehavior,
       elevation: elevation,
       color: ElevationOverlay.applyOverlay(context, widget.color, elevation),
-      shadowColor: _shadowColor.evaluate(animation),
+      shadowColor: _shadowColor!.evaluate(animation!)!,
     );
   }
 }
 
 class _ShapeBorderPaint extends StatelessWidget {
   const _ShapeBorderPaint({
-    @required this.child,
-    @required this.shape,
+    required this.child,
+    required this.shape,
     this.borderOnForeground = true,
   });
 
@@ -826,7 +822,7 @@ class _ShapeBorderPaint extends StatelessWidget {
 class _ShapeBorderPainter extends CustomPainter {
   _ShapeBorderPainter(this.border, this.textDirection);
   final ShapeBorder border;
-  final TextDirection textDirection;
+  final TextDirection? textDirection;
 
   @override
   void paint(Canvas canvas, Size size) {

packages/flutter/lib/src/material/material_localizations.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.
 
-// @dart = 2.8
-
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
@@ -135,7 +133,7 @@ abstract class MaterialLocalizations {
   /// there are, e.g. 'Tab 1 of 2' in United States English.
   ///
   /// `tabIndex` and `tabCount` must be greater than or equal to one.
-  String tabLabel({ int tabIndex, int tabCount });
+  String tabLabel({ required int tabIndex, required int tabCount });
 
   /// Title for the [PaginatedDataTable]'s selected row count header.
   String selectedRowCountTitle(int selectedRowCount);
@@ -319,7 +317,7 @@ abstract class MaterialLocalizations {
   ///
   /// See also:
   ///   * [formatCompactDate], which will convert a [DateTime] into a string in the compact format.
-  DateTime parseCompactDate(String inputString);
+  DateTime? parseCompactDate(String? inputString);
 
   /// List of week day names in narrow format, usually 1- or 2-letter
   /// abbreviations of full names.
@@ -505,7 +503,7 @@ abstract class MaterialLocalizations {
   /// ```dart
   /// tooltip: MaterialLocalizations.of(context).backButtonTooltip,
   /// ```
-  static MaterialLocalizations of(BuildContext context) {
+  static MaterialLocalizations? of(BuildContext context) {
     return Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);
   }
 }
@@ -696,24 +694,28 @@ class DefaultMaterialLocalizations implements MaterialLocalizations {
   }
 
   @override
-  DateTime parseCompactDate(String inputString) {
+  DateTime? parseCompactDate(String? inputString) {
+    if (inputString == null) {
+      return null;
+    }
+
     // Assumes US mm/dd/yyyy format
     final List<String> inputParts = inputString.split('/');
     if (inputParts.length != 3) {
       return null;
     }
 
-    final int year = int.tryParse(inputParts[2], radix: 10);
+    final int? year = int.tryParse(inputParts[2], radix: 10);
     if (year == null || year < 1) {
       return null;
     }
 
-    final int month = int.tryParse(inputParts[0], radix: 10);
+    final int? month = int.tryParse(inputParts[0], radix: 10);
     if (month == null || month < 1 || month > 12) {
       return null;
     }
 
-    final int day = int.tryParse(inputParts[1], radix: 10);
+    final int? day = int.tryParse(inputParts[1], radix: 10);
     if (day == null || day < 1 || day > _getDaysInMonth(year, month)) {
       return null;
     }
@@ -808,7 +810,6 @@ class DefaultMaterialLocalizations implements MaterialLocalizations {
       case DayPeriod.pm:
         return postMeridiemAbbreviation;
     }
-    return null;
   }
 
   @override
@@ -932,7 +933,7 @@ class DefaultMaterialLocalizations implements MaterialLocalizations {
   String get rowsPerPageTitle => 'Rows per page:';
 
   @override
-  String tabLabel({ int tabIndex, int tabCount }) {
+  String tabLabel({ required int tabIndex, required int tabCount }) {
     assert(tabIndex >= 1);
     assert(tabCount >= 1);
     return 'Tab $tabIndex of $tabCount';

packages/flutter/lib/src/material/snack_bar.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.
 
-// @dart = 2.8
-
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
@@ -85,22 +83,22 @@ class SnackBarAction extends StatefulWidget {
   ///
   /// The [label] and [onPressed] arguments must be non-null.
   const SnackBarAction({
-    Key key,
+    Key? key,
     this.textColor,
     this.disabledTextColor,
-    @required this.label,
-    @required this.onPressed,
+    required this.label,
+    required this.onPressed,
   }) : assert(label != null),
        assert(onPressed != null),
        super(key: key);
 
   /// The button label color. If not provided, defaults to
   /// [SnackBarThemeData.actionTextColor].
-  final Color textColor;
+  final Color? textColor;
 
   /// The button disabled label color. This color is shown after the
   /// [SnackBarAction] is dismissed.
-  final Color disabledTextColor;
+  final Color? disabledTextColor;
 
   /// The button label.
   final String label;
@@ -125,13 +123,13 @@ class _SnackBarActionState extends State<SnackBarAction> {
       _haveTriggeredAction = true;
     });
     widget.onPressed();
-    Scaffold.of(context).hideCurrentSnackBar(reason: SnackBarClosedReason.action);
+    Scaffold.of(context)!.hideCurrentSnackBar(reason: SnackBarClosedReason.action);
   }
 
   @override
   Widget build(BuildContext context) {
-    Color resolveForegroundColor(Set<MaterialState> states) {
-      final SnackBarThemeData snackBarTheme = Theme.of(context).snackBarTheme;
+    Color? resolveForegroundColor(Set<MaterialState> states) {
+      final SnackBarThemeData snackBarTheme = Theme.of(context)!.snackBarTheme;
       if (states.contains(MaterialState.disabled))
         return widget.disabledTextColor ?? snackBarTheme.disabledActionTextColor;
       return widget.textColor ?? snackBarTheme.actionTextColor;
@@ -178,8 +176,8 @@ class SnackBar extends StatefulWidget {
   /// The [content] argument must be non-null. The [elevation] must be null or
   /// non-negative.
   const SnackBar({
-    Key key,
-    @required this.content,
+    Key? key,
+    required this.content,
     this.backgroundColor,
     this.elevation,
     this.margin,
@@ -218,7 +216,7 @@ class SnackBar extends StatefulWidget {
   /// is not specified it will default to a dark variation of
   /// [ColorScheme.surface] for light themes, or [ColorScheme.onSurface] for
   /// dark themes.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// The z-coordinate at which to place the snack bar. This controls the size
   /// of the shadow below the snack bar.
@@ -228,7 +226,7 @@ class SnackBar extends StatefulWidget {
   /// If this property is null, then [SnackBarThemeData.elevation] of
   /// [ThemeData.snackBarTheme] is used, if that is also null, the default value
   /// is 6.0.
-  final double elevation;
+  final double? elevation;
 
   /// Empty space to surround the snack bar.
   ///
@@ -237,7 +235,7 @@ class SnackBar extends StatefulWidget {
   ///
   /// If this property is null, then the default is
   /// `EdgeInsets.fromLTRB(15.0, 5.0, 15.0, 10.0)`.
-  final EdgeInsetsGeometry margin;
+  final EdgeInsetsGeometry? margin;
 
   /// The amount of padding to apply to the snack bar's content and optional
   /// action.
@@ -246,7 +244,7 @@ class SnackBar extends StatefulWidget {
   /// the presence of an [action]. The start padding is 24 if [behavior] is
   /// [SnackBarBehavior.fixed] and 16 if it is [SnackBarBehavior.floating]. If
   /// there is no [action], the same padding is added to the end.
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   /// The width of the snack bar.
   ///
@@ -256,7 +254,7 @@ class SnackBar extends StatefulWidget {
   ///
   /// If this property is null, then the snack bar will take up the full device
   /// width less the margin.
-  final double width;
+  final double? width;
 
   /// The shape of the snack bar's [Material].
   ///
@@ -268,7 +266,7 @@ class SnackBar extends StatefulWidget {
   /// overriding shape is specified, so the [SnackBar] is rectangular. For
   /// [SnackBarBehavior.floating], it uses a [RoundedRectangleBorder] with a
   /// circular corner radius of 4.0.
-  final ShapeBorder shape;
+  final ShapeBorder? shape;
 
   /// This defines the behavior and location of the snack bar.
   ///
@@ -279,7 +277,7 @@ class SnackBar extends StatefulWidget {
   /// If this property is null, then [SnackBarThemeData.behavior] of
   /// [ThemeData.snackBarTheme] is used. If that is null, then the default is
   /// [SnackBarBehavior.fixed].
-  final SnackBarBehavior behavior;
+  final SnackBarBehavior? behavior;
 
   /// (optional) An action that the user can take based on the snack bar.
   ///
@@ -287,7 +285,7 @@ class SnackBar extends StatefulWidget {
   /// prompted the snackbar. Snack bars can have at most one action.
   ///
   /// The action should not be "dismiss" or "cancel".
-  final SnackBarAction action;
+  final SnackBarAction? action;
 
   /// The amount of time the snack bar should be displayed.
   ///
@@ -302,15 +300,15 @@ class SnackBar extends StatefulWidget {
   final Duration duration;
 
   /// The animation driving the entrance and exit of the snack bar.
-  final Animation<double> animation;
+  final Animation<double>? animation;
 
   /// Called the first time that the snackbar is visible within a [Scaffold].
-  final VoidCallback onVisible;
+  final VoidCallback? onVisible;
 
   // API for Scaffold.showSnackBar():
 
   /// Creates an animation controller useful for driving a snack bar's entrance and exit animation.
-  static AnimationController createAnimationController({ @required TickerProvider vsync }) {
+  static AnimationController createAnimationController({ required TickerProvider vsync }) {
     return AnimationController(
       duration: _snackBarTransitionDuration,
       debugLabel: 'SnackBar',
@@ -322,7 +320,7 @@ class SnackBar extends StatefulWidget {
   ///
   /// If the original snack bar lacks a key, the newly created snack bar will
   /// use the given fallback key.
-  SnackBar withAnimation(Animation<double> newAnimation, { Key fallbackKey }) {
+  SnackBar withAnimation(Animation<double> newAnimation, { Key? fallbackKey }) {
     return SnackBar(
       key: key ?? fallbackKey,
       content: content,
@@ -351,21 +349,21 @@ class _SnackBarState extends State<SnackBar> {
   @override
   void initState() {
     super.initState();
-    widget.animation.addStatusListener(_onAnimationStatusChanged);
+    widget.animation!.addStatusListener(_onAnimationStatusChanged);
   }
 
   @override
   void didUpdateWidget(SnackBar oldWidget) {
     if (widget.animation != oldWidget.animation) {
-      oldWidget.animation.removeStatusListener(_onAnimationStatusChanged);
-      widget.animation.addStatusListener(_onAnimationStatusChanged);
+      oldWidget.animation!.removeStatusListener(_onAnimationStatusChanged);
+      widget.animation!.addStatusListener(_onAnimationStatusChanged);
     }
     super.didUpdateWidget(oldWidget);
   }
 
   @override
   void dispose() {
-    widget.animation.removeStatusListener(_onAnimationStatusChanged);
+    widget.animation!.removeStatusListener(_onAnimationStatusChanged);
     super.dispose();
   }
 
@@ -377,7 +375,7 @@ class _SnackBarState extends State<SnackBar> {
         break;
       case AnimationStatus.completed:
         if (widget.onVisible != null && !_wasVisible) {
-          widget.onVisible();
+          widget.onVisible!();
         }
         _wasVisible = true;
     }
@@ -385,9 +383,10 @@ class _SnackBarState extends State<SnackBar> {
 
   @override
   Widget build(BuildContext context) {
-    final MediaQueryData mediaQueryData = MediaQuery.of(context);
+    assert(debugCheckHasMediaQuery(context));
+    final MediaQueryData mediaQueryData = MediaQuery.of(context)!;
     assert(widget.animation != null);
-    final ThemeData theme = Theme.of(context);
+    final ThemeData theme = Theme.of(context)!;
     final ColorScheme colorScheme = theme.colorScheme;
     final SnackBarThemeData snackBarTheme = theme.snackBarTheme;
     final bool isThemeDark = theme.brightness == Brightness.dark;
@@ -422,17 +421,17 @@ class _SnackBarState extends State<SnackBar> {
       snackBarTheme: snackBarTheme,
     );
 
-    final TextStyle contentTextStyle = snackBarTheme.contentTextStyle ?? inverseTheme.textTheme.subtitle1;
+    final TextStyle? contentTextStyle = snackBarTheme.contentTextStyle ?? inverseTheme.textTheme.subtitle1;
     final SnackBarBehavior snackBarBehavior = widget.behavior ?? snackBarTheme.behavior ?? SnackBarBehavior.fixed;
     final bool isFloatingSnackBar = snackBarBehavior == SnackBarBehavior.floating;
     final double horizontalPadding = isFloatingSnackBar ? 16.0 : 24.0;
     final EdgeInsetsGeometry padding = widget.padding
       ?? EdgeInsetsDirectional.only(start: horizontalPadding, end: widget.action != null ? 0 : horizontalPadding);
 
-    final CurvedAnimation heightAnimation = CurvedAnimation(parent: widget.animation, curve: _snackBarHeightCurve);
-    final CurvedAnimation fadeInAnimation = CurvedAnimation(parent: widget.animation, curve: _snackBarFadeInCurve);
+    final CurvedAnimation heightAnimation = CurvedAnimation(parent: widget.animation!, curve: _snackBarHeightCurve);
+    final CurvedAnimation fadeInAnimation = CurvedAnimation(parent: widget.animation!, curve: _snackBarFadeInCurve);
     final CurvedAnimation fadeOutAnimation = CurvedAnimation(
-      parent: widget.animation,
+      parent: widget.animation!,
       curve: _snackBarFadeOutCurve,
       reverseCurve: const Threshold(0.0),
     );
@@ -474,7 +473,7 @@ class _SnackBarState extends State<SnackBar> {
 
     final double elevation = widget.elevation ?? snackBarTheme.elevation ?? 6.0;
     final Color backgroundColor = widget.backgroundColor ?? snackBarTheme.backgroundColor ?? inverseTheme.backgroundColor;
-    final ShapeBorder shape = widget.shape
+    final ShapeBorder? shape = widget.shape
       ?? snackBarTheme.shape
       ?? (isFloatingSnackBar ? RoundedRectangleBorder(borderRadius: BorderRadius.circular(4.0)) : null);
 
@@ -526,14 +525,14 @@ class _SnackBarState extends State<SnackBar> {
       container: true,
       liveRegion: true,
       onDismiss: () {
-        Scaffold.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.dismiss);
+        Scaffold.of(context)!.removeCurrentSnackBar(reason: SnackBarClosedReason.dismiss);
       },
       child: Dismissible(
         key: const Key('dismissible'),
         direction: DismissDirection.down,
         resizeDuration: null,
         onDismissed: (DismissDirection direction) {
-          Scaffold.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.swipe);
+          Scaffold.of(context)!.removeCurrentSnackBar(reason: SnackBarClosedReason.swipe);
         },
         child: snackBar,
       ),
@@ -550,7 +549,7 @@ class _SnackBarState extends State<SnackBar> {
     } else {
       snackBarTransition = AnimatedBuilder(
         animation: heightAnimation,
-        builder: (BuildContext context, Widget child) {
+        builder: (BuildContext context, Widget? child) {
           return Align(
             alignment: AlignmentDirectional.topStart,
             heightFactor: heightAnimation.value,

packages/flutter/lib/src/material/snack_bar_theme.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.
 
-// @dart = 2.8
-
 import 'dart:ui' show lerpDouble;
 
 import 'package:flutter/foundation.dart';
@@ -67,30 +65,30 @@ class SnackBarThemeData with Diagnosticable {
   /// Default value for [SnackBar.backgroundColor].
   ///
   /// If null, [SnackBar] defaults to dark grey: `Color(0xFF323232)`.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// Default value for [SnackBarAction.textColor].
   ///
   /// If null, [SnackBarAction] defaults to [ColorScheme.secondary] of
   /// [ThemeData.colorScheme] .
-  final Color actionTextColor;
+  final Color? actionTextColor;
 
   /// Default value for [SnackBarAction.disabledTextColor].
   ///
   /// If null, [SnackBarAction] defaults to [ColorScheme.onSurface] with its
   /// opacity set to 0.30 if the [Theme]'s brightness is [Brightness.dark], 0.38
   /// otherwise.
-  final Color disabledActionTextColor;
+  final Color? disabledActionTextColor;
 
   /// Used to configure the [DefaultTextStyle] for the [SnackBar.content] widget.
   ///
   /// If null, [SnackBar] defines its default.
-  final TextStyle contentTextStyle;
+  final TextStyle? contentTextStyle;
 
   /// Default value for [SnackBar.elevation].
   ///
   /// If null, [SnackBar] uses a default of 6.0.
-  final double elevation;
+  final double? elevation;
 
   /// Default value for [SnackBar.shape].
   ///
@@ -99,23 +97,23 @@ class SnackBarThemeData with Diagnosticable {
   /// specified, so the [SnackBar] is rectangular. For
   /// [SnackBarBehavior.floating], it uses a [RoundedRectangleBorder] with a
   /// circular corner radius of 4.0.
-  final ShapeBorder shape;
+  final ShapeBorder? shape;
 
   /// Default value for [SnackBar.behavior].
   ///
   /// If null, [SnackBar] will default to [SnackBarBehavior.fixed].
-  final SnackBarBehavior behavior;
+  final SnackBarBehavior? behavior;
 
   /// Creates a copy of this object with the given fields replaced with the
   /// new values.
   SnackBarThemeData copyWith({
-    Color backgroundColor,
-    Color actionTextColor,
-    Color disabledActionTextColor,
-    TextStyle contentTextStyle,
-    double elevation,
-    ShapeBorder shape,
-    SnackBarBehavior behavior,
+    Color? backgroundColor,
+    Color? actionTextColor,
+    Color? disabledActionTextColor,
+    TextStyle? contentTextStyle,
+    double? elevation,
+    ShapeBorder? shape,
+    SnackBarBehavior? behavior,
   }) {
     return SnackBarThemeData(
       backgroundColor: backgroundColor ?? this.backgroundColor,
@@ -133,7 +131,7 @@ class SnackBarThemeData with Diagnosticable {
   /// The argument `t` must not be null.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static SnackBarThemeData lerp(SnackBarThemeData a, SnackBarThemeData b, double t) {
+  static SnackBarThemeData lerp(SnackBarThemeData? a, SnackBarThemeData? b, double t) {
     assert(t != null);
     return SnackBarThemeData(
       backgroundColor: Color.lerp(a?.backgroundColor, b?.backgroundColor, t),
@@ -142,7 +140,7 @@ class SnackBarThemeData with Diagnosticable {
       contentTextStyle: TextStyle.lerp(a?.contentTextStyle, b?.contentTextStyle, t),
       elevation: lerpDouble(a?.elevation, b?.elevation, t),
       shape: ShapeBorder.lerp(a?.shape, b?.shape, t),
-      behavior: t < 0.5 ? a.behavior : b.behavior,
+      behavior: t < 0.5 ? a?.behavior : b?.behavior,
     );
   }
 

packages/flutter/lib/src/material/theme.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.
 
-// @dart = 2.8
-
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
@@ -40,10 +38,10 @@ class Theme extends StatelessWidget {
   ///
   /// The [data] and [child] arguments must not be null.
   const Theme({
-    Key key,
-    @required this.data,
+    Key? key,
+    required this.data,
     this.isMaterialAppTheme = false,
-    @required this.child,
+    required this.child,
   }) : assert(child != null),
        assert(data != null),
        super(key: key);
@@ -126,17 +124,17 @@ class Theme extends StatelessWidget {
   ///   );
   /// }
   /// ```
-  static ThemeData of(BuildContext context, { bool shadowThemeOnly = false }) {
-    final _InheritedTheme inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedTheme>();
+  static ThemeData? of(BuildContext context, { bool shadowThemeOnly = false }) {
+    final _InheritedTheme? inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedTheme>();
     if (shadowThemeOnly) {
       if (inheritedTheme == null || inheritedTheme.theme.isMaterialAppTheme)
         return null;
       return inheritedTheme.theme.data;
     }
 
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
+    final MaterialLocalizations? localizations = MaterialLocalizations.of(context);
     final ScriptCategory category = localizations?.scriptCategory ?? ScriptCategory.englishLike;
-    final ThemeData theme = inheritedTheme?.theme?.data ?? _kFallbackTheme;
+    final ThemeData theme = inheritedTheme?.theme.data ?? _kFallbackTheme;
     return ThemeData.localize(theme, theme.typography.geometryThemeFor(category));
   }
 
@@ -168,9 +166,9 @@ class Theme extends StatelessWidget {
 
 class _InheritedTheme extends InheritedTheme {
   const _InheritedTheme({
-    Key key,
-    @required this.theme,
-    @required Widget child,
+    Key? key,
+    required this.theme,
+    required Widget child,
   }) : assert(theme != null),
        super(key: key, child: child);
 
@@ -178,7 +176,7 @@ class _InheritedTheme extends InheritedTheme {
 
   @override
   Widget wrap(BuildContext context, Widget child) {
-    final _InheritedTheme ancestorTheme = context.findAncestorWidgetOfExactType<_InheritedTheme>();
+    final _InheritedTheme? ancestorTheme = context.findAncestorWidgetOfExactType<_InheritedTheme>();
     return identical(this, ancestorTheme) ? child : Theme(data: theme.data, child: child);
   }
 
@@ -198,10 +196,10 @@ class ThemeDataTween extends Tween<ThemeData> {
   /// The [begin] and [end] properties must be non-null before the tween is
   /// first used, but the arguments can be null if the values are going to be
   /// filled in later.
-  ThemeDataTween({ ThemeData begin, ThemeData end }) : super(begin: begin, end: end);
+  ThemeDataTween({ ThemeData? begin, ThemeData? end }) : super(begin: begin, end: end);
 
   @override
-  ThemeData lerp(double t) => ThemeData.lerp(begin, end, t);
+  ThemeData lerp(double t) => ThemeData.lerp(begin!, end!, t);
 }
 
 /// Animated version of [Theme] which automatically transitions the colors,
@@ -224,13 +222,13 @@ class AnimatedTheme extends ImplicitlyAnimatedWidget {
   /// By default, the theme transition uses a linear curve. The [data] and
   /// [child] arguments must not be null.
   const AnimatedTheme({
-    Key key,
-    @required this.data,
+    Key? key,
+    required this.data,
     this.isMaterialAppTheme = false,
     Curve curve = Curves.linear,
     Duration duration = kThemeAnimationDuration,
-    VoidCallback onEnd,
-    @required this.child,
+    VoidCallback? onEnd,
+    required this.child,
   }) : assert(child != null),
        assert(data != null),
        super(key: key, curve: curve, duration: duration, onEnd: onEnd);
@@ -251,7 +249,7 @@ class AnimatedTheme extends ImplicitlyAnimatedWidget {
 }
 
 class _AnimatedThemeState extends AnimatedWidgetBaseState<AnimatedTheme> {
-  ThemeDataTween _data;
+  ThemeDataTween? _data;
 
   @override
   void forEachTween(TweenVisitor<dynamic> visitor) {
@@ -265,7 +263,7 @@ class _AnimatedThemeState extends AnimatedWidgetBaseState<AnimatedTheme> {
     return Theme(
       isMaterialAppTheme: widget.isMaterialAppTheme,
       child: widget.child,
-      data: _data.evaluate(animation),
+      data: _data!.evaluate(animation!),
     );
   }
 

packages/flutter/lib/src/material/theme_data.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.
 
-// @dart = 2.8
-
 import 'dart:ui' show Color, hashList, lerpDouble;
 
 import 'package:flutter/cupertino.dart';
@@ -211,118 +209,118 @@ class ThemeData with Diagnosticable {
   ///  * [ThemeData.light], which creates a light blue theme.
   ///  * [ThemeData.dark], which creates dark theme with a teal secondary [ColorScheme] color.
   factory ThemeData({
-    Brightness brightness,
-    VisualDensity visualDensity,
-    MaterialColor primarySwatch,
-    Color primaryColor,
-    Brightness primaryColorBrightness,
-    Color primaryColorLight,
-    Color primaryColorDark,
-    Color accentColor,
-    Brightness accentColorBrightness,
-    Color canvasColor,
-    Color shadowColor,
-    Color scaffoldBackgroundColor,
-    Color bottomAppBarColor,
-    Color cardColor,
-    Color dividerColor,
-    Color focusColor,
-    Color hoverColor,
-    Color highlightColor,
-    Color splashColor,
-    InteractiveInkFeatureFactory splashFactory,
-    Color selectedRowColor,
-    Color unselectedWidgetColor,
-    Color disabledColor,
-    Color buttonColor,
-    ButtonThemeData buttonTheme,
-    ToggleButtonsThemeData toggleButtonsTheme,
-    Color secondaryHeaderColor,
+    Brightness? brightness,
+    VisualDensity? visualDensity,
+    MaterialColor? primarySwatch,
+    Color? primaryColor,
+    Brightness? primaryColorBrightness,
+    Color? primaryColorLight,
+    Color? primaryColorDark,
+    Color? accentColor,
+    Brightness? accentColorBrightness,
+    Color? canvasColor,
+    Color? shadowColor,
+    Color? scaffoldBackgroundColor,
+    Color? bottomAppBarColor,
+    Color? cardColor,
+    Color? dividerColor,
+    Color? focusColor,
+    Color? hoverColor,
+    Color? highlightColor,
+    Color? splashColor,
+    InteractiveInkFeatureFactory? splashFactory,
+    Color? selectedRowColor,
+    Color? unselectedWidgetColor,
+    Color? disabledColor,
+    Color? buttonColor,
+    ButtonThemeData? buttonTheme,
+    ToggleButtonsThemeData? toggleButtonsTheme,
+    Color? secondaryHeaderColor,
     @Deprecated(
       'Use TextSelectionThemeData.selectionColor instead. '
       'This feature was deprecated after v1.23.0-4.0.pre.'
     )
-    Color textSelectionColor,
+    Color? textSelectionColor,
     @Deprecated(
       'Use TextSelectionThemeData.cursorColor instead. '
       'This feature was deprecated after v1.23.0-4.0.pre.'
     )
-    Color cursorColor,
+    Color? cursorColor,
     @Deprecated(
       'Use TextSelectionThemeData.selectionHandleColor instead. '
       'This feature was deprecated after v1.23.0-4.0.pre.'
     )
-    Color textSelectionHandleColor,
-    Color backgroundColor,
-    Color dialogBackgroundColor,
-    Color indicatorColor,
-    Color hintColor,
-    Color errorColor,
-    Color toggleableActiveColor,
-    String fontFamily,
-    TextTheme textTheme,
-    TextTheme primaryTextTheme,
-    TextTheme accentTextTheme,
-    InputDecorationTheme inputDecorationTheme,
-    IconThemeData iconTheme,
-    IconThemeData primaryIconTheme,
-    IconThemeData accentIconTheme,
-    SliderThemeData sliderTheme,
-    TabBarTheme tabBarTheme,
-    TooltipThemeData tooltipTheme,
-    CardTheme cardTheme,
-    ChipThemeData chipTheme,
-    TargetPlatform platform,
-    MaterialTapTargetSize materialTapTargetSize,
-    bool applyElevationOverlayColor,
-    PageTransitionsTheme pageTransitionsTheme,
-    AppBarTheme appBarTheme,
-    BottomAppBarTheme bottomAppBarTheme,
-    ColorScheme colorScheme,
-    DialogTheme dialogTheme,
-    FloatingActionButtonThemeData floatingActionButtonTheme,
-    NavigationRailThemeData navigationRailTheme,
-    Typography typography,
-    NoDefaultCupertinoThemeData cupertinoOverrideTheme,
-    SnackBarThemeData snackBarTheme,
-    BottomSheetThemeData bottomSheetTheme,
-    PopupMenuThemeData popupMenuTheme,
-    MaterialBannerThemeData bannerTheme,
-    DividerThemeData dividerTheme,
-    ButtonBarThemeData buttonBarTheme,
-    BottomNavigationBarThemeData bottomNavigationBarTheme,
-    TimePickerThemeData timePickerTheme,
-    TextButtonThemeData textButtonTheme,
-    ElevatedButtonThemeData elevatedButtonTheme,
-    OutlinedButtonThemeData outlinedButtonTheme,
-    TextSelectionThemeData textSelectionTheme,
-    DataTableThemeData dataTableTheme,
-    bool fixTextFieldOutlineLabel,
+    Color? textSelectionHandleColor,
+    Color? backgroundColor,
+    Color? dialogBackgroundColor,
+    Color? indicatorColor,
+    Color? hintColor,
+    Color? errorColor,
+    Color? toggleableActiveColor,
+    String? fontFamily,
+    TextTheme? textTheme,
+    TextTheme? primaryTextTheme,
+    TextTheme? accentTextTheme,
+    InputDecorationTheme? inputDecorationTheme,
+    IconThemeData? iconTheme,
+    IconThemeData? primaryIconTheme,
+    IconThemeData? accentIconTheme,
+    SliderThemeData? sliderTheme,
+    TabBarTheme? tabBarTheme,
+    TooltipThemeData? tooltipTheme,
+    CardTheme? cardTheme,
+    ChipThemeData? chipTheme,
+    TargetPlatform? platform,
+    MaterialTapTargetSize? materialTapTargetSize,
+    bool? applyElevationOverlayColor,
+    PageTransitionsTheme? pageTransitionsTheme,
+    AppBarTheme? appBarTheme,
+    BottomAppBarTheme? bottomAppBarTheme,
+    ColorScheme? colorScheme,
+    DialogTheme? dialogTheme,
+    FloatingActionButtonThemeData? floatingActionButtonTheme,
+    NavigationRailThemeData? navigationRailTheme,
+    Typography? typography,
+    NoDefaultCupertinoThemeData? cupertinoOverrideTheme,
+    SnackBarThemeData? snackBarTheme,
+    BottomSheetThemeData? bottomSheetTheme,
+    PopupMenuThemeData? popupMenuTheme,
+    MaterialBannerThemeData? bannerTheme,
+    DividerThemeData? dividerTheme,
+    ButtonBarThemeData? buttonBarTheme,
+    BottomNavigationBarThemeData? bottomNavigationBarTheme,
+    TimePickerThemeData? timePickerTheme,
+    TextButtonThemeData? textButtonTheme,
+    ElevatedButtonThemeData? elevatedButtonTheme,
+    OutlinedButtonThemeData? outlinedButtonTheme,
+    TextSelectionThemeData? textSelectionTheme,
+    DataTableThemeData? dataTableTheme,
+    bool? fixTextFieldOutlineLabel,
     @Deprecated(
       'No longer used by the framework, please remove any reference to it. '
       'This feature was deprecated after v1.23.0-4.0.pre.'
     )
-    bool useTextSelectionTheme,
+    bool? useTextSelectionTheme,
   }) {
-    assert(colorScheme?.brightness == null || brightness == null || colorScheme.brightness == brightness);
+    assert(colorScheme?.brightness == null || brightness == null || colorScheme!.brightness == brightness);
     final Brightness _brightness = brightness ?? colorScheme?.brightness ?? Brightness.light;
     final bool isDark = _brightness == Brightness.dark;
     visualDensity ??= VisualDensity.adaptivePlatformDensity;
     primarySwatch ??= Colors.blue;
-    primaryColor ??= isDark ? Colors.grey[900] : primarySwatch;
+    primaryColor ??= isDark ? Colors.grey[900]! : primarySwatch;
     primaryColorBrightness ??= estimateBrightnessForColor(primaryColor);
-    primaryColorLight ??= isDark ? Colors.grey[500] : primarySwatch[100];
-    primaryColorDark ??= isDark ? Colors.black : primarySwatch[700];
+    primaryColorLight ??= isDark ? Colors.grey[500]! : primarySwatch[100]!;
+    primaryColorDark ??= isDark ? Colors.black : primarySwatch[700]!;
     final bool primaryIsDark = primaryColorBrightness == Brightness.dark;
-    toggleableActiveColor ??= isDark ? Colors.tealAccent[200] : (accentColor ?? primarySwatch[600]);
-    accentColor ??= isDark ? Colors.tealAccent[200] : primarySwatch[500];
+    toggleableActiveColor ??= isDark ? Colors.tealAccent[200]! : (accentColor ?? primarySwatch[600]!);
+    accentColor ??= isDark ? Colors.tealAccent[200]! : primarySwatch[500]!;
     accentColorBrightness ??= estimateBrightnessForColor(accentColor);
     final bool accentIsDark = accentColorBrightness == Brightness.dark;
-    canvasColor ??= isDark ? Colors.grey[850] : Colors.grey[50];
+    canvasColor ??= isDark ? Colors.grey[850]! : Colors.grey[50]!;
     shadowColor ??= Colors.black;
     scaffoldBackgroundColor ??= canvasColor;
-    bottomAppBarColor ??= isDark ? Colors.grey[800] : Colors.white;
-    cardColor ??= isDark ? Colors.grey[800] : Colors.white;
+    bottomAppBarColor ??= isDark ? Colors.grey[800]! : Colors.white;
+    cardColor ??= isDark ? Colors.grey[800]! : Colors.white;
     dividerColor ??= isDark ? const Color(0x1FFFFFFF) : const Color(0x1F000000);
 
     // Create a ColorScheme that is backwards compatible as possible
@@ -338,18 +336,18 @@ class ThemeData with Diagnosticable {
     );
 
     splashFactory ??= InkSplash.splashFactory;
-    selectedRowColor ??= Colors.grey[100];
+    selectedRowColor ??= Colors.grey[100]!;
     unselectedWidgetColor ??= isDark ? Colors.white70 : Colors.black54;
     // Spec doesn't specify a dark theme secondaryHeaderColor, this is a guess.
-    secondaryHeaderColor ??= isDark ? Colors.grey[700] : primarySwatch[50];
-    textSelectionColor ??= isDark ? accentColor : primarySwatch[200];
+    secondaryHeaderColor ??= isDark ? Colors.grey[700]! : primarySwatch[50]!;
+    textSelectionColor ??= isDark ? accentColor : primarySwatch[200]!;
     cursorColor = cursorColor ?? const Color.fromRGBO(66, 133, 244, 1.0);
-    textSelectionHandleColor ??= isDark ? Colors.tealAccent[400] : primarySwatch[300];
-    backgroundColor ??= isDark ? Colors.grey[700] : primarySwatch[200];
-    dialogBackgroundColor ??= isDark ? Colors.grey[800] : Colors.white;
+    textSelectionHandleColor ??= isDark ? Colors.tealAccent[400]! : primarySwatch[300]!;
+    backgroundColor ??= isDark ? Colors.grey[700]! : primarySwatch[200]!;
+    dialogBackgroundColor ??= isDark ? Colors.grey[800]! : Colors.white;
     indicatorColor ??= accentColor == primaryColor ? Colors.white : accentColor;
     hintColor ??= isDark ? Colors.white60 : Colors.black.withOpacity(0.6);
-    errorColor ??= Colors.red[700];
+    errorColor ??= Colors.red[700]!;
     inputDecorationTheme ??= const InputDecorationTheme();
     pageTransitionsTheme ??= const PageTransitionsTheme();
     primaryIconTheme ??= primaryIsDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
@@ -384,7 +382,7 @@ class ThemeData with Diagnosticable {
 
     // Used as the default color (fill color) for RaisedButtons. Computing the
     // default for ButtonThemeData for the sake of backwards compatibility.
-    buttonColor ??= isDark ? primarySwatch[600] : Colors.grey[300];
+    buttonColor ??= isDark ? primarySwatch[600]! : Colors.grey[300]!;
     focusColor ??= isDark ? Colors.white.withOpacity(0.12) : Colors.black.withOpacity(0.12);
     hoverColor ??= isDark ? Colors.white.withOpacity(0.04) : Colors.black.withOpacity(0.04);
     buttonTheme ??= ButtonThemeData(
@@ -411,7 +409,7 @@ class ThemeData with Diagnosticable {
     chipTheme ??= ChipThemeData.fromDefaults(
       secondaryColor: primaryColor,
       brightness: colorScheme.brightness,
-      labelStyle: textTheme.bodyText1,
+      labelStyle: textTheme.bodyText1!,
     );
     dialogTheme ??= const DialogTheme();
     floatingActionButtonTheme ??= const FloatingActionButtonThemeData();
@@ -522,79 +520,79 @@ class ThemeData with Diagnosticable {
     // Warning: make sure these properties are in the exact same order as in
     // operator == and in the hashValues method and in the order of fields
     // in this class, and in the lerp() method.
-    @required this.visualDensity,
-    @required this.primaryColor,
-    @required this.primaryColorBrightness,
-    @required this.primaryColorLight,
-    @required this.primaryColorDark,
-    @required this.canvasColor,
-    @required this.shadowColor,
-    @required this.accentColor,
-    @required this.accentColorBrightness,
-    @required this.scaffoldBackgroundColor,
-    @required this.bottomAppBarColor,
-    @required this.cardColor,
-    @required this.dividerColor,
-    @required this.focusColor,
-    @required this.hoverColor,
-    @required this.highlightColor,
-    @required this.splashColor,
-    @required this.splashFactory,
-    @required this.selectedRowColor,
-    @required this.unselectedWidgetColor,
-    @required this.disabledColor,
-    @required this.buttonTheme,
-    @required this.buttonColor,
-    @required this.toggleButtonsTheme,
-    @required this.secondaryHeaderColor,
-    @required this.textSelectionColor,
-    @required this.cursorColor,
-    @required this.textSelectionHandleColor,
-    @required this.backgroundColor,
-    @required this.dialogBackgroundColor,
-    @required this.indicatorColor,
-    @required this.hintColor,
-    @required this.errorColor,
-    @required this.toggleableActiveColor,
-    @required this.textTheme,
-    @required this.primaryTextTheme,
-    @required this.accentTextTheme,
-    @required this.inputDecorationTheme,
-    @required this.iconTheme,
-    @required this.primaryIconTheme,
-    @required this.accentIconTheme,
-    @required this.sliderTheme,
-    @required this.tabBarTheme,
-    @required this.tooltipTheme,
-    @required this.cardTheme,
-    @required this.chipTheme,
-    @required this.platform,
-    @required this.materialTapTargetSize,
-    @required this.applyElevationOverlayColor,
-    @required this.pageTransitionsTheme,
-    @required this.appBarTheme,
-    @required this.bottomAppBarTheme,
-    @required this.colorScheme,
-    @required this.dialogTheme,
-    @required this.floatingActionButtonTheme,
-    @required this.navigationRailTheme,
-    @required this.typography,
-    @required this.cupertinoOverrideTheme,
-    @required this.snackBarTheme,
-    @required this.bottomSheetTheme,
-    @required this.popupMenuTheme,
-    @required this.bannerTheme,
-    @required this.dividerTheme,
-    @required this.buttonBarTheme,
-    @required this.bottomNavigationBarTheme,
-    @required this.timePickerTheme,
-    @required this.textButtonTheme,
-    @required this.elevatedButtonTheme,
-    @required this.outlinedButtonTheme,
-    @required this.textSelectionTheme,
-    @required this.dataTableTheme,
-    @required this.fixTextFieldOutlineLabel,
-    @required this.useTextSelectionTheme,
+    required this.visualDensity,
+    required this.primaryColor,
+    required this.primaryColorBrightness,
+    required this.primaryColorLight,
+    required this.primaryColorDark,
+    required this.canvasColor,
+    required this.shadowColor,
+    required this.accentColor,
+    required this.accentColorBrightness,
+    required this.scaffoldBackgroundColor,
+    required this.bottomAppBarColor,
+    required this.cardColor,
+    required this.dividerColor,
+    required this.focusColor,
+    required this.hoverColor,
+    required this.highlightColor,
+    required this.splashColor,
+    required this.splashFactory,
+    required this.selectedRowColor,
+    required this.unselectedWidgetColor,
+    required this.disabledColor,
+    required this.buttonTheme,
+    required this.buttonColor,
+    required this.toggleButtonsTheme,
+    required this.secondaryHeaderColor,
+    required this.textSelectionColor,
+    required this.cursorColor,
+    required this.textSelectionHandleColor,
+    required this.backgroundColor,
+    required this.dialogBackgroundColor,
+    required this.indicatorColor,
+    required this.hintColor,
+    required this.errorColor,
+    required this.toggleableActiveColor,
+    required this.textTheme,
+    required this.primaryTextTheme,
+    required this.accentTextTheme,
+    required this.inputDecorationTheme,
+    required this.iconTheme,
+    required this.primaryIconTheme,
+    required this.accentIconTheme,
+    required this.sliderTheme,
+    required this.tabBarTheme,
+    required this.tooltipTheme,
+    required this.cardTheme,
+    required this.chipTheme,
+    required this.platform,
+    required this.materialTapTargetSize,
+    required this.applyElevationOverlayColor,
+    required this.pageTransitionsTheme,
+    required this.appBarTheme,
+    required this.bottomAppBarTheme,
+    required this.colorScheme,
+    required this.dialogTheme,
+    required this.floatingActionButtonTheme,
+    required this.navigationRailTheme,
+    required this.typography,
+    required this.cupertinoOverrideTheme,
+    required this.snackBarTheme,
+    required this.bottomSheetTheme,
+    required this.popupMenuTheme,
+    required this.bannerTheme,
+    required this.dividerTheme,
+    required this.buttonBarTheme,
+    required this.bottomNavigationBarTheme,
+    required this.timePickerTheme,
+    required this.textButtonTheme,
+    required this.elevatedButtonTheme,
+    required this.outlinedButtonTheme,
+    required this.textSelectionTheme,
+    required this.dataTableTheme,
+    required this.fixTextFieldOutlineLabel,
+    required this.useTextSelectionTheme,
   }) : assert(visualDensity != null),
        assert(primaryColor != null),
        assert(primaryColorBrightness != null),
@@ -695,8 +693,8 @@ class ThemeData with Diagnosticable {
   /// See <https://material.io/design/color/> for
   /// more discussion on how to pick the right colors.
   factory ThemeData.from({
-    @required ColorScheme colorScheme,
-    TextTheme textTheme,
+    required ColorScheme colorScheme,
+    TextTheme? textTheme,
   }) {
     assert(colorScheme != null);
 
@@ -1110,7 +1108,7 @@ class ThemeData with Diagnosticable {
   ///
   /// This cascading effect for individual attributes of the [CupertinoThemeData]
   /// can be overridden using attributes of this [cupertinoOverrideTheme].
-  final NoDefaultCupertinoThemeData cupertinoOverrideTheme;
+  final NoDefaultCupertinoThemeData? cupertinoOverrideTheme;
 
   /// A theme for customizing the color, elevation, and shape of a bottom sheet.
   final BottomSheetThemeData bottomSheetTheme;
@@ -1181,96 +1179,96 @@ class ThemeData with Diagnosticable {
   ///
   /// The [brightness] value is applied to the [colorScheme].
   ThemeData copyWith({
-    Brightness brightness,
-    VisualDensity visualDensity,
-    Color primaryColor,
-    Brightness primaryColorBrightness,
-    Color primaryColorLight,
-    Color primaryColorDark,
-    Color accentColor,
-    Brightness accentColorBrightness,
-    Color canvasColor,
-    Color shadowColor,
-    Color scaffoldBackgroundColor,
-    Color bottomAppBarColor,
-    Color cardColor,
-    Color dividerColor,
-    Color focusColor,
-    Color hoverColor,
-    Color highlightColor,
-    Color splashColor,
-    InteractiveInkFeatureFactory splashFactory,
-    Color selectedRowColor,
-    Color unselectedWidgetColor,
-    Color disabledColor,
-    ButtonThemeData buttonTheme,
-    ToggleButtonsThemeData toggleButtonsTheme,
-    Color buttonColor,
-    Color secondaryHeaderColor,
+    Brightness? brightness,
+    VisualDensity? visualDensity,
+    Color? primaryColor,
+    Brightness? primaryColorBrightness,
+    Color? primaryColorLight,
+    Color? primaryColorDark,
+    Color? accentColor,
+    Brightness? accentColorBrightness,
+    Color? canvasColor,
+    Color? shadowColor,
+    Color? scaffoldBackgroundColor,
+    Color? bottomAppBarColor,
+    Color? cardColor,
+    Color? dividerColor,
+    Color? focusColor,
+    Color? hoverColor,
+    Color? highlightColor,
+    Color? splashColor,
+    InteractiveInkFeatureFactory? splashFactory,
+    Color? selectedRowColor,
+    Color? unselectedWidgetColor,
+    Color? disabledColor,
+    ButtonThemeData? buttonTheme,
+    ToggleButtonsThemeData? toggleButtonsTheme,
+    Color? buttonColor,
+    Color? secondaryHeaderColor,
     @Deprecated(
       'Use TextSelectionThemeData.selectionColor instead. '
       'This feature was deprecated after v1.23.0-4.0.pre.'
     )
-    Color textSelectionColor,
+    Color? textSelectionColor,
     @Deprecated(
       'Use TextSelectionThemeData.cursorColor instead. '
       'This feature was deprecated after v1.23.0-4.0.pre.'
     )
-    Color cursorColor,
+    Color? cursorColor,
     @Deprecated(
       'Use TextSelectionThemeData.selectionHandleColor instead. '
       'This feature was deprecated after v1.23.0-4.0.pre.'
     )
-    Color textSelectionHandleColor,
-    Color backgroundColor,
-    Color dialogBackgroundColor,
-    Color indicatorColor,
-    Color hintColor,
-    Color errorColor,
-    Color toggleableActiveColor,
-    TextTheme textTheme,
-    TextTheme primaryTextTheme,
-    TextTheme accentTextTheme,
-    InputDecorationTheme inputDecorationTheme,
-    IconThemeData iconTheme,
-    IconThemeData primaryIconTheme,
-    IconThemeData accentIconTheme,
-    SliderThemeData sliderTheme,
-    TabBarTheme tabBarTheme,
-    TooltipThemeData tooltipTheme,
-    CardTheme cardTheme,
-    ChipThemeData chipTheme,
-    TargetPlatform platform,
-    MaterialTapTargetSize materialTapTargetSize,
-    bool applyElevationOverlayColor,
-    PageTransitionsTheme pageTransitionsTheme,
-    AppBarTheme appBarTheme,
-    BottomAppBarTheme bottomAppBarTheme,
-    ColorScheme colorScheme,
-    DialogTheme dialogTheme,
-    FloatingActionButtonThemeData floatingActionButtonTheme,
-    NavigationRailThemeData navigationRailTheme,
-    Typography typography,
-    NoDefaultCupertinoThemeData cupertinoOverrideTheme,
-    SnackBarThemeData snackBarTheme,
-    BottomSheetThemeData bottomSheetTheme,
-    PopupMenuThemeData popupMenuTheme,
-    MaterialBannerThemeData bannerTheme,
-    DividerThemeData dividerTheme,
-    ButtonBarThemeData buttonBarTheme,
-    BottomNavigationBarThemeData bottomNavigationBarTheme,
-    TimePickerThemeData timePickerTheme,
-    TextButtonThemeData textButtonTheme,
-    ElevatedButtonThemeData elevatedButtonTheme,
-    OutlinedButtonThemeData outlinedButtonTheme,
-    TextSelectionThemeData textSelectionTheme,
-    DataTableThemeData dataTableTheme,
-    bool fixTextFieldOutlineLabel,
+    Color? textSelectionHandleColor,
+    Color? backgroundColor,
+    Color? dialogBackgroundColor,
+    Color? indicatorColor,
+    Color? hintColor,
+    Color? errorColor,
+    Color? toggleableActiveColor,
+    TextTheme? textTheme,
+    TextTheme? primaryTextTheme,
+    TextTheme? accentTextTheme,
+    InputDecorationTheme? inputDecorationTheme,
+    IconThemeData? iconTheme,
+    IconThemeData? primaryIconTheme,
+    IconThemeData? accentIconTheme,
+    SliderThemeData? sliderTheme,
+    TabBarTheme? tabBarTheme,
+    TooltipThemeData? tooltipTheme,
+    CardTheme? cardTheme,
+    ChipThemeData? chipTheme,
+    TargetPlatform? platform,
+    MaterialTapTargetSize? materialTapTargetSize,
+    bool? applyElevationOverlayColor,
+    PageTransitionsTheme? pageTransitionsTheme,
+    AppBarTheme? appBarTheme,
+    BottomAppBarTheme? bottomAppBarTheme,
+    ColorScheme? colorScheme,
+    DialogTheme? dialogTheme,
+    FloatingActionButtonThemeData? floatingActionButtonTheme,
+    NavigationRailThemeData? navigationRailTheme,
+    Typography? typography,
+    NoDefaultCupertinoThemeData? cupertinoOverrideTheme,
+    SnackBarThemeData? snackBarTheme,
+    BottomSheetThemeData? bottomSheetTheme,
+    PopupMenuThemeData? popupMenuTheme,
+    MaterialBannerThemeData? bannerTheme,
+    DividerThemeData? dividerTheme,
+    ButtonBarThemeData? buttonBarTheme,
+    BottomNavigationBarThemeData? bottomNavigationBarTheme,
+    TimePickerThemeData? timePickerTheme,
+    TextButtonThemeData? textButtonTheme,
+    ElevatedButtonThemeData? elevatedButtonTheme,
+    OutlinedButtonThemeData? outlinedButtonTheme,
+    TextSelectionThemeData? textSelectionTheme,
+    DataTableThemeData? dataTableTheme,
+    bool? fixTextFieldOutlineLabel,
     @Deprecated(
       'No longer used by the framework, please remove any reference to it. '
       'This feature was deprecated after v1.23.0-4.0.pre.'
     )
-    bool useTextSelectionTheme,
+    bool? useTextSelectionTheme,
   }) {
     cupertinoOverrideTheme = cupertinoOverrideTheme?.noDefault();
     return ThemeData.raw(
@@ -1429,39 +1427,39 @@ class ThemeData with Diagnosticable {
     // the class and in the lerp() method.
     return ThemeData.raw(
       visualDensity: VisualDensity.lerp(a.visualDensity, b.visualDensity, t),
-      primaryColor: Color.lerp(a.primaryColor, b.primaryColor, t),
+      primaryColor: Color.lerp(a.primaryColor, b.primaryColor, t)!,
       primaryColorBrightness: t < 0.5 ? a.primaryColorBrightness : b.primaryColorBrightness,
-      primaryColorLight: Color.lerp(a.primaryColorLight, b.primaryColorLight, t),
-      primaryColorDark: Color.lerp(a.primaryColorDark, b.primaryColorDark, t),
-      canvasColor: Color.lerp(a.canvasColor, b.canvasColor, t),
-      shadowColor: Color.lerp(a.shadowColor, b.shadowColor, t),
-      accentColor: Color.lerp(a.accentColor, b.accentColor, t),
+      primaryColorLight: Color.lerp(a.primaryColorLight, b.primaryColorLight, t)!,
+      primaryColorDark: Color.lerp(a.primaryColorDark, b.primaryColorDark, t)!,
+      canvasColor: Color.lerp(a.canvasColor, b.canvasColor, t)!,
+      shadowColor: Color.lerp(a.shadowColor, b.shadowColor, t)!,
+      accentColor: Color.lerp(a.accentColor, b.accentColor, t)!,
       accentColorBrightness: t < 0.5 ? a.accentColorBrightness : b.accentColorBrightness,
-      scaffoldBackgroundColor: Color.lerp(a.scaffoldBackgroundColor, b.scaffoldBackgroundColor, t),
-      bottomAppBarColor: Color.lerp(a.bottomAppBarColor, b.bottomAppBarColor, t),
-      cardColor: Color.lerp(a.cardColor, b.cardColor, t),
-      dividerColor: Color.lerp(a.dividerColor, b.dividerColor, t),
-      focusColor: Color.lerp(a.focusColor, b.focusColor, t),
-      hoverColor: Color.lerp(a.hoverColor, b.hoverColor, t),
-      highlightColor: Color.lerp(a.highlightColor, b.highlightColor, t),
-      splashColor: Color.lerp(a.splashColor, b.splashColor, t),
+      scaffoldBackgroundColor: Color.lerp(a.scaffoldBackgroundColor, b.scaffoldBackgroundColor, t)!,
+      bottomAppBarColor: Color.lerp(a.bottomAppBarColor, b.bottomAppBarColor, t)!,
+      cardColor: Color.lerp(a.cardColor, b.cardColor, t)!,
+      dividerColor: Color.lerp(a.dividerColor, b.dividerColor, t)!,
+      focusColor: Color.lerp(a.focusColor, b.focusColor, t)!,
+      hoverColor: Color.lerp(a.hoverColor, b.hoverColor, t)!,
+      highlightColor: Color.lerp(a.highlightColor, b.highlightColor, t)!,
+      splashColor: Color.lerp(a.splashColor, b.splashColor, t)!,
       splashFactory: t < 0.5 ? a.splashFactory : b.splashFactory,
-      selectedRowColor: Color.lerp(a.selectedRowColor, b.selectedRowColor, t),
-      unselectedWidgetColor: Color.lerp(a.unselectedWidgetColor, b.unselectedWidgetColor, t),
-      disabledColor: Color.lerp(a.disabledColor, b.disabledColor, t),
+      selectedRowColor: Color.lerp(a.selectedRowColor, b.selectedRowColor, t)!,
+      unselectedWidgetColor: Color.lerp(a.unselectedWidgetColor, b.unselectedWidgetColor, t)!,
+      disabledColor: Color.lerp(a.disabledColor, b.disabledColor, t)!,
       buttonTheme: t < 0.5 ? a.buttonTheme : b.buttonTheme,
       toggleButtonsTheme: ToggleButtonsThemeData.lerp(a.toggleButtonsTheme, b.toggleButtonsTheme, t),
-      buttonColor: Color.lerp(a.buttonColor, b.buttonColor, t),
-      secondaryHeaderColor: Color.lerp(a.secondaryHeaderColor, b.secondaryHeaderColor, t),
-      textSelectionColor: Color.lerp(a.textSelectionColor, b.textSelectionColor, t),
-      cursorColor: Color.lerp(a.cursorColor, b.cursorColor, t),
-      textSelectionHandleColor: Color.lerp(a.textSelectionHandleColor, b.textSelectionHandleColor, t),
-      backgroundColor: Color.lerp(a.backgroundColor, b.backgroundColor, t),
-      dialogBackgroundColor: Color.lerp(a.dialogBackgroundColor, b.dialogBackgroundColor, t),
-      indicatorColor: Color.lerp(a.indicatorColor, b.indicatorColor, t),
-      hintColor: Color.lerp(a.hintColor, b.hintColor, t),
-      errorColor: Color.lerp(a.errorColor, b.errorColor, t),
-      toggleableActiveColor: Color.lerp(a.toggleableActiveColor, b.toggleableActiveColor, t),
+      buttonColor: Color.lerp(a.buttonColor, b.buttonColor, t)!,
+      secondaryHeaderColor: Color.lerp(a.secondaryHeaderColor, b.secondaryHeaderColor, t)!,
+      textSelectionColor: Color.lerp(a.textSelectionColor, b.textSelectionColor, t)!,
+      cursorColor: Color.lerp(a.cursorColor, b.cursorColor, t)!,
+      textSelectionHandleColor: Color.lerp(a.textSelectionHandleColor, b.textSelectionHandleColor, t)!,
+      backgroundColor: Color.lerp(a.backgroundColor, b.backgroundColor, t)!,
+      dialogBackgroundColor: Color.lerp(a.dialogBackgroundColor, b.dialogBackgroundColor, t)!,
+      indicatorColor: Color.lerp(a.indicatorColor, b.indicatorColor, t)!,
+      hintColor: Color.lerp(a.hintColor, b.hintColor, t)!,
+      errorColor: Color.lerp(a.errorColor, b.errorColor, t)!,
+      toggleableActiveColor: Color.lerp(a.toggleableActiveColor, b.toggleableActiveColor, t)!,
       textTheme: TextTheme.lerp(a.textTheme, b.textTheme, t),
       primaryTextTheme: TextTheme.lerp(a.primaryTextTheme, b.primaryTextTheme, t),
       accentTextTheme: TextTheme.lerp(a.accentTextTheme, b.accentTextTheme, t),
@@ -1471,9 +1469,9 @@ class ThemeData with Diagnosticable {
       accentIconTheme: IconThemeData.lerp(a.accentIconTheme, b.accentIconTheme, t),
       sliderTheme: SliderThemeData.lerp(a.sliderTheme, b.sliderTheme, t),
       tabBarTheme: TabBarTheme.lerp(a.tabBarTheme, b.tabBarTheme, t),
-      tooltipTheme: TooltipThemeData.lerp(a.tooltipTheme, b.tooltipTheme, t),
+      tooltipTheme: TooltipThemeData.lerp(a.tooltipTheme, b.tooltipTheme, t)!,
       cardTheme: CardTheme.lerp(a.cardTheme, b.cardTheme, t),
-      chipTheme: ChipThemeData.lerp(a.chipTheme, b.chipTheme, t),
+      chipTheme: ChipThemeData.lerp(a.chipTheme, b.chipTheme, t)!,
       platform: t < 0.5 ? a.platform : b.platform,
       materialTapTargetSize: t < 0.5 ? a.materialTapTargetSize : b.materialTapTargetSize,
       applyElevationOverlayColor: t < 0.5 ? a.applyElevationOverlayColor : b.applyElevationOverlayColor,
@@ -1482,20 +1480,20 @@ class ThemeData with Diagnosticable {
       bottomAppBarTheme: BottomAppBarTheme.lerp(a.bottomAppBarTheme, b.bottomAppBarTheme, t),
       colorScheme: ColorScheme.lerp(a.colorScheme, b.colorScheme, t),
       dialogTheme: DialogTheme.lerp(a.dialogTheme, b.dialogTheme, t),
-      floatingActionButtonTheme: FloatingActionButtonThemeData.lerp(a.floatingActionButtonTheme, b.floatingActionButtonTheme, t),
+      floatingActionButtonTheme: FloatingActionButtonThemeData.lerp(a.floatingActionButtonTheme, b.floatingActionButtonTheme, t)!,
       navigationRailTheme: NavigationRailThemeData.lerp(a.navigationRailTheme, b.navigationRailTheme, t),
       typography: Typography.lerp(a.typography, b.typography, t),
       cupertinoOverrideTheme: t < 0.5 ? a.cupertinoOverrideTheme : b.cupertinoOverrideTheme,
       snackBarTheme: SnackBarThemeData.lerp(a.snackBarTheme, b.snackBarTheme, t),
-      bottomSheetTheme: BottomSheetThemeData.lerp(a.bottomSheetTheme, b.bottomSheetTheme, t),
+      bottomSheetTheme: BottomSheetThemeData.lerp(a.bottomSheetTheme, b.bottomSheetTheme, t)!,
       popupMenuTheme: PopupMenuThemeData.lerp(a.popupMenuTheme, b.popupMenuTheme, t),
       bannerTheme: MaterialBannerThemeData.lerp(a.bannerTheme, b.bannerTheme, t),
       dividerTheme: DividerThemeData.lerp(a.dividerTheme, b.dividerTheme, t),
-      buttonBarTheme: ButtonBarThemeData.lerp(a.buttonBarTheme, b.buttonBarTheme, t),
+      buttonBarTheme: ButtonBarThemeData.lerp(a.buttonBarTheme, b.buttonBarTheme, t)!,
       bottomNavigationBarTheme: BottomNavigationBarThemeData.lerp(a.bottomNavigationBarTheme, b.bottomNavigationBarTheme, t),
       timePickerTheme: TimePickerThemeData.lerp(a.timePickerTheme, b.timePickerTheme, t),
       textButtonTheme: TextButtonThemeData.lerp(a.textButtonTheme, b.textButtonTheme, t),
-      elevatedButtonTheme: ElevatedButtonThemeData.lerp(a.elevatedButtonTheme, b.elevatedButtonTheme, t),
+      elevatedButtonTheme: ElevatedButtonThemeData.lerp(a.elevatedButtonTheme, b.elevatedButtonTheme, t)!,
       outlinedButtonTheme: OutlinedButtonThemeData.lerp(a.outlinedButtonTheme, b.outlinedButtonTheme, t),
       textSelectionTheme: TextSelectionThemeData .lerp(a.textSelectionTheme, b.textSelectionTheme, t),
       dataTableTheme: DataTableThemeData.lerp(a.dataTableTheme, b.dataTableTheme, t),
@@ -1590,7 +1588,7 @@ class ThemeData with Diagnosticable {
     // Warning: For the sanity of the reader, please make sure these properties
     // are in the exact same order as in operator == and in the raw constructor
     // and in the order of fields in the class and in the lerp() method.
-    final List<Object> values = <Object>[
+    final List<Object?> values = <Object?>[
       visualDensity,
       primaryColor,
       primaryColorBrightness,
@@ -1779,7 +1777,7 @@ class MaterialBasedCupertinoThemeData extends CupertinoThemeData {
   ///
   /// The [materialTheme] parameter must not be null.
   MaterialBasedCupertinoThemeData({
-    @required ThemeData materialTheme,
+    required ThemeData materialTheme,
   }) : this._(
     materialTheme,
     (materialTheme.cupertinoOverrideTheme ?? const CupertinoThemeData()).noDefault(),
@@ -1830,12 +1828,12 @@ class MaterialBasedCupertinoThemeData extends CupertinoThemeData {
   /// instead.
   @override
   MaterialBasedCupertinoThemeData copyWith({
-    Brightness brightness,
-    Color primaryColor,
-    Color primaryContrastingColor,
-    CupertinoTextThemeData textTheme,
-    Color barBackgroundColor,
-    Color scaffoldBackgroundColor,
+    Brightness? brightness,
+    Color? primaryColor,
+    Color? primaryContrastingColor,
+    CupertinoTextThemeData? textTheme,
+    Color? barBackgroundColor,
+    Color? scaffoldBackgroundColor,
   }) {
     return MaterialBasedCupertinoThemeData._(
       _materialTheme,
@@ -1909,7 +1907,7 @@ class _FifoCache<K, V> {
   V putIfAbsent(K key, V loader()) {
     assert(key != null);
     assert(loader != null);
-    final V result = _cache[key];
+    final V? result = _cache[key];
     if (result != null)
       return result;
     if (_cache.length == _maximumSize)
@@ -2010,8 +2008,8 @@ class VisualDensity with Diagnosticable {
   /// Copy the current [VisualDensity] with the given values replacing the
   /// current values.
   VisualDensity copyWith({
-    double horizontal,
-    double vertical,
+    double? horizontal,
+    double? vertical,
   }) {
     return VisualDensity(
       horizontal: horizontal ?? this.horizontal,
@@ -2070,8 +2068,8 @@ class VisualDensity with Diagnosticable {
   /// Linearly interpolate between two densities.
   static VisualDensity lerp(VisualDensity a, VisualDensity b, double t) {
     return VisualDensity(
-      horizontal: lerpDouble(a.horizontal, b.horizontal, t),
-      vertical: lerpDouble(a.horizontal, b.horizontal, t),
+      horizontal: lerpDouble(a.horizontal, b.horizontal, t)!,
+      vertical: lerpDouble(a.horizontal, b.horizontal, t)!,
     );
   }
 

packages/flutter/lib/src/material/time.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.
 
-// @dart = 2.8
-
 import 'dart:ui' show hashValues;
 
 import 'package:flutter/widgets.dart';
@@ -53,7 +51,7 @@ class TimeOfDay {
   ///
   /// The [hour] argument must be between 0 and 23, inclusive. The [minute]
   /// argument must be between 0 and 59, inclusive.
-  const TimeOfDay({ @required this.hour, @required this.minute });
+  const TimeOfDay({ required this.hour, required this.minute });
 
   /// Creates a time of day based on the given time.
   ///
@@ -79,7 +77,7 @@ class TimeOfDay {
   static const int minutesPerHour = 60;
 
   /// Returns a new TimeOfDay with the hour and/or minute replaced.
-  TimeOfDay replacing({ int hour, int minute }) {
+  TimeOfDay replacing({ int? hour, int? minute }) {
     assert(hour == null || (hour >= 0 && hour < hoursPerDay));
     assert(minute == null || (minute >= 0 && minute < minutesPerHour));
     return TimeOfDay(hour: hour ?? this.hour, minute: minute ?? this.minute);
@@ -106,10 +104,10 @@ class TimeOfDay {
   String format(BuildContext context) {
     assert(debugCheckHasMediaQuery(context));
     assert(debugCheckHasMaterialLocalizations(context));
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
     return localizations.formatTimeOfDay(
       this,
-      alwaysUse24HourFormat: MediaQuery.of(context).alwaysUse24HourFormat,
+      alwaysUse24HourFormat: MediaQuery.of(context)!.alwaysUse24HourFormat,
     );
   }
 
@@ -204,7 +202,7 @@ enum HourFormat {
 }
 
 /// The [HourFormat] used for the given [TimeOfDayFormat].
-HourFormat hourFormat({ @required TimeOfDayFormat of }) {
+HourFormat hourFormat({ required TimeOfDayFormat of }) {
   switch (of) {
     case TimeOfDayFormat.h_colon_mm_space_a:
     case TimeOfDayFormat.a_space_h_colon_mm:
@@ -216,6 +214,4 @@ HourFormat hourFormat({ @required TimeOfDayFormat of }) {
     case TimeOfDayFormat.frenchCanadian:
       return HourFormat.HH;
   }
-
-  return null;
 }

packages/flutter/lib/src/material/tooltip.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.
 
-// @dart = 2.8
-
 import 'dart:async';
 
 import 'package:flutter/gestures.dart';
@@ -49,8 +47,8 @@ class Tooltip extends StatefulWidget {
   /// All parameters that are defined in the constructor will
   /// override the default values _and_ the values in [TooltipTheme.of].
   const Tooltip({
-    Key key,
-    @required this.message,
+    Key? key,
+    required this.message,
     this.height,
     this.padding,
     this.margin,
@@ -71,12 +69,12 @@ class Tooltip extends StatefulWidget {
   /// The height of the tooltip's [child].
   ///
   /// If the [child] is null, then this is the tooltip's intrinsic height.
-  final double height;
+  final double? height;
 
   /// The amount of space by which to inset the tooltip's [child].
   ///
   /// Defaults to 16.0 logical pixels in each direction.
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   /// The empty space that surrounds the tooltip.
   ///
@@ -89,7 +87,7 @@ class Tooltip extends StatefulWidget {
   /// If this property is null, then [TooltipThemeData.margin] is used.
   /// If [TooltipThemeData.margin] is also null, the default margin is
   /// 0.0 logical pixels on all sides.
-  final EdgeInsetsGeometry margin;
+  final EdgeInsetsGeometry? margin;
 
   /// The vertical gap between the widget and the displayed tooltip.
   ///
@@ -98,14 +96,14 @@ class Tooltip extends StatefulWidget {
   /// tooltips will position themselves under their corresponding widgets.
   /// Otherwise, tooltips will position themselves above their corresponding
   /// widgets with the given offset.
-  final double verticalOffset;
+  final double? verticalOffset;
 
   /// Whether the tooltip defaults to being displayed below the widget.
   ///
   /// Defaults to true. If there is insufficient space to display the tooltip in
   /// the preferred direction, the tooltip will be displayed in the opposite
   /// direction.
-  final bool preferBelow;
+  final bool? preferBelow;
 
   /// Whether the tooltip's [message] should be excluded from the semantics
   /// tree.
@@ -113,12 +111,12 @@ class Tooltip extends StatefulWidget {
   /// Defaults to false. A tooltip will add a [Semantics] label that is set to
   /// [Tooltip.message]. Set this property to true if the app is going to
   /// provide its own custom semantics label.
-  final bool excludeFromSemantics;
+  final bool? excludeFromSemantics;
 
   /// The widget below this widget in the tree.
   ///
   /// {@macro flutter.widgets.child}
-  final Widget child;
+  final Widget? child;
 
   /// Specifies the tooltip's shape and background color.
   ///
@@ -126,7 +124,7 @@ class Tooltip extends StatefulWidget {
   /// 4.0. Tooltips will also default to an opacity of 90% and with the color
   /// [Colors.grey[700]] if [ThemeData.brightness] is [Brightness.dark], and
   /// [Colors.white] if it is [Brightness.light].
-  final Decoration decoration;
+  final Decoration? decoration;
 
   /// The style to use for the message of the tooltip.
   ///
@@ -136,7 +134,7 @@ class Tooltip extends StatefulWidget {
   /// [Colors.white]. Otherwise, if [ThemeData.brightness] is set to
   /// [Brightness.light], [TextTheme.bodyText2] of [ThemeData.textTheme] will be
   /// used with [Colors.black].
-  final TextStyle textStyle;
+  final TextStyle? textStyle;
 
   /// The length of time that a pointer must hover over a tooltip's widget
   /// before the tooltip will be shown.
@@ -145,13 +143,13 @@ class Tooltip extends StatefulWidget {
   /// disappear.
   ///
   /// Defaults to 0 milliseconds (tooltips are shown immediately upon hover).
-  final Duration waitDuration;
+  final Duration? waitDuration;
 
   /// The length of time that the tooltip will be shown after a long press
   /// is released.
   ///
   /// Defaults to 1.5 seconds.
-  final Duration showDuration;
+  final Duration? showDuration;
 
   @override
   _TooltipState createState() => _TooltipState();
@@ -183,27 +181,27 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
   static const Duration _defaultWaitDuration = Duration(milliseconds: 0);
   static const bool _defaultExcludeFromSemantics = false;
 
-  double height;
-  EdgeInsetsGeometry padding;
-  EdgeInsetsGeometry margin;
-  Decoration decoration;
-  TextStyle textStyle;
-  double verticalOffset;
-  bool preferBelow;
-  bool excludeFromSemantics;
-  AnimationController _controller;
-  OverlayEntry _entry;
-  Timer _hideTimer;
-  Timer _showTimer;
-  Duration showDuration;
-  Duration waitDuration;
-  bool _mouseIsConnected;
+  late double height;
+  late EdgeInsetsGeometry padding;
+  late EdgeInsetsGeometry margin;
+  late Decoration decoration;
+  late TextStyle textStyle;
+  late double verticalOffset;
+  late bool preferBelow;
+  late bool excludeFromSemantics;
+  late AnimationController _controller;
+  OverlayEntry? _entry;
+  Timer? _hideTimer;
+  Timer? _showTimer;
+  late Duration showDuration;
+  late Duration waitDuration;
+  late bool _mouseIsConnected;
   bool _longPressActivated = false;
 
   @override
   void initState() {
     super.initState();
-    _mouseIsConnected = RendererBinding.instance.mouseTracker.mouseIsConnected;
+    _mouseIsConnected = RendererBinding.instance!.mouseTracker.mouseIsConnected;
     _controller = AnimationController(
       duration: _fadeInDuration,
       reverseDuration: _fadeOutDuration,
@@ -211,10 +209,10 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
     )
       ..addStatusListener(_handleStatusChanged);
     // Listen to see when a mouse is added.
-    RendererBinding.instance.mouseTracker.addListener(_handleMouseTrackerChange);
+    RendererBinding.instance!.mouseTracker.addListener(_handleMouseTrackerChange);
     // Listen to global pointer events so that we can hide a tooltip immediately
     // if some other control is clicked on.
-    GestureBinding.instance.pointerRouter.addGlobalRoute(_handlePointerEvent);
+    GestureBinding.instance!.pointerRouter.addGlobalRoute(_handlePointerEvent);
   }
 
   // Forces a rebuild if a mouse has been added or removed.
@@ -222,7 +220,7 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
     if (!mounted) {
       return;
     }
-    final bool mouseIsConnected = RendererBinding.instance.mouseTracker.mouseIsConnected;
+    final bool mouseIsConnected = RendererBinding.instance!.mouseTracker.mouseIsConnected;
     if (mouseIsConnected != _mouseIsConnected) {
       setState((){
         _mouseIsConnected = mouseIsConnected;
@@ -287,9 +285,9 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
     final OverlayState overlayState = Overlay.of(
       context,
       debugRequiredFor: widget,
-    );
+    )!;
 
-    final RenderBox box = context.findRenderObject() as RenderBox;
+    final RenderBox box = context.findRenderObject()! as RenderBox;
     final Offset target = box.localToGlobal(
       box.size.center(Offset.zero),
       ancestor: overlayState.context.findRenderObject(),
@@ -299,7 +297,7 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
     // updated values from happening to leak into the overlay when the overlay
     // rebuilds.
     final Widget overlay = Directionality(
-      textDirection: Directionality.of(context),
+      textDirection: Directionality.of(context)!,
       child: _TooltipOverlay(
         message: widget.message,
         height: height,
@@ -317,7 +315,7 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
       ),
     );
     _entry = OverlayEntry(builder: (BuildContext context) => overlay);
-    overlayState.insert(_entry);
+    overlayState.insert(_entry!);
     SemanticsService.tooltip(widget.message);
   }
 
@@ -352,8 +350,8 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
 
   @override
   void dispose() {
-    GestureBinding.instance.pointerRouter.removeGlobalRoute(_handlePointerEvent);
-    RendererBinding.instance.mouseTracker.removeListener(_handleMouseTrackerChange);
+    GestureBinding.instance!.pointerRouter.removeGlobalRoute(_handlePointerEvent);
+    RendererBinding.instance!.mouseTracker.removeListener(_handleMouseTrackerChange);
     if (_entry != null)
       _removeEntry();
     _controller.dispose();
@@ -370,12 +368,12 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
   @override
   Widget build(BuildContext context) {
     assert(Overlay.of(context, debugRequiredFor: widget) != null);
-    final ThemeData theme = Theme.of(context);
+    final ThemeData theme = Theme.of(context)!;
     final TooltipThemeData tooltipTheme = TooltipTheme.of(context);
     TextStyle defaultTextStyle;
     BoxDecoration defaultDecoration;
     if (theme.brightness == Brightness.dark) {
-      defaultTextStyle = theme.textTheme.bodyText2.copyWith(
+      defaultTextStyle = theme.textTheme.bodyText2!.copyWith(
         color: Colors.black,
       );
       defaultDecoration = BoxDecoration(
@@ -383,11 +381,11 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
         borderRadius: const BorderRadius.all(Radius.circular(4)),
       );
     } else {
-      defaultTextStyle = theme.textTheme.bodyText2.copyWith(
+      defaultTextStyle = theme.textTheme.bodyText2!.copyWith(
         color: Colors.white,
       );
       defaultDecoration = BoxDecoration(
-        color: Colors.grey[700].withOpacity(0.9),
+        color: Colors.grey[700]!.withOpacity(0.9),
         borderRadius: const BorderRadius.all(Radius.circular(4)),
       );
     }
@@ -433,9 +431,9 @@ class _TooltipPositionDelegate extends SingleChildLayoutDelegate {
   ///
   /// The arguments must not be null.
   _TooltipPositionDelegate({
-    @required this.target,
-    @required this.verticalOffset,
-    @required this.preferBelow,
+    required this.target,
+    required this.verticalOffset,
+    required this.preferBelow,
   }) : assert(target != null),
        assert(verticalOffset != null),
        assert(preferBelow != null);
@@ -478,25 +476,25 @@ class _TooltipPositionDelegate extends SingleChildLayoutDelegate {
 
 class _TooltipOverlay extends StatelessWidget {
   const _TooltipOverlay({
-    Key key,
-    this.message,
-    this.height,
+    Key? key,
+    required this.message,
+    required this.height,
     this.padding,
     this.margin,
     this.decoration,
     this.textStyle,
-    this.animation,
-    this.target,
-    this.verticalOffset,
-    this.preferBelow,
+    required this.animation,
+    required this.target,
+    required this.verticalOffset,
+    required this.preferBelow,
   }) : super(key: key);
 
   final String message;
   final double height;
-  final EdgeInsetsGeometry padding;
-  final EdgeInsetsGeometry margin;
-  final Decoration decoration;
-  final TextStyle textStyle;
+  final EdgeInsetsGeometry? padding;
+  final EdgeInsetsGeometry? margin;
+  final Decoration? decoration;
+  final TextStyle? textStyle;
   final Animation<double> animation;
   final Offset target;
   final double verticalOffset;
@@ -517,7 +515,7 @@ class _TooltipOverlay extends StatelessWidget {
             child: ConstrainedBox(
               constraints: BoxConstraints(minHeight: height),
               child: DefaultTextStyle(
-                style: Theme.of(context).textTheme.bodyText2,
+                style: Theme.of(context)!.textTheme.bodyText2,
                 child: Container(
                   decoration: decoration,
                   padding: padding,

packages/flutter/lib/src/material/tooltip_theme.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.
 
-// @dart = 2.8
-
 import 'dart:ui' show lerpDouble;
 
 import 'package:flutter/foundation.dart';
@@ -42,13 +40,13 @@ class TooltipThemeData with Diagnosticable {
   });
 
   /// The height of [Tooltip.child].
-  final double height;
+  final double? height;
 
   /// If provided, the amount of space by which to inset [Tooltip.child].
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   /// If provided, the amount of empty space to surround the [Tooltip].
-  final EdgeInsetsGeometry margin;
+  final EdgeInsetsGeometry? margin;
 
   /// The vertical gap between the widget and the displayed tooltip.
   ///
@@ -57,13 +55,13 @@ class TooltipThemeData with Diagnosticable {
   /// tooltips will position themselves under their corresponding widgets.
   /// Otherwise, tooltips will position themselves above their corresponding
   /// widgets with the given offset.
-  final double verticalOffset;
+  final double? verticalOffset;
 
   /// Whether the tooltip is displayed below its widget by default.
   ///
   /// If there is insufficient space to display the tooltip in the preferred
   /// direction, the tooltip will be displayed in the opposite direction.
-  final bool preferBelow;
+  final bool? preferBelow;
 
   /// Whether the [Tooltip.message] should be excluded from the semantics
   /// tree.
@@ -71,34 +69,34 @@ class TooltipThemeData with Diagnosticable {
   /// By default, [Tooltip]s will add a [Semantics] label that is set to
   /// [Tooltip.message]. Set this property to true if the app is going to
   /// provide its own custom semantics label.
-  final bool excludeFromSemantics;
+  final bool? excludeFromSemantics;
 
   /// The [Tooltip]'s shape and background color.
-  final Decoration decoration;
+  final Decoration? decoration;
 
   /// The style to use for the message of [Tooltip]s.
-  final TextStyle textStyle;
+  final TextStyle? textStyle;
 
   /// The length of time that a pointer must hover over a tooltip's widget
   /// before the tooltip will be shown.
-  final Duration waitDuration;
+  final Duration? waitDuration;
 
   /// The length of time that the tooltip will be shown once it has appeared.
-  final Duration showDuration;
+  final Duration? showDuration;
 
   /// Creates a copy of this object but with the given fields replaced with the
   /// new values.
   TooltipThemeData copyWith({
-    double height,
-    EdgeInsetsGeometry padding,
-    EdgeInsetsGeometry margin,
-    double verticalOffset,
-    bool preferBelow,
-    bool excludeFromSemantics,
-    Decoration decoration,
-    TextStyle textStyle,
-    Duration waitDuration,
-    Duration showDuration,
+    double? height,
+    EdgeInsetsGeometry? padding,
+    EdgeInsetsGeometry? margin,
+    double? verticalOffset,
+    bool? preferBelow,
+    bool? excludeFromSemantics,
+    Decoration? decoration,
+    TextStyle? textStyle,
+    Duration? waitDuration,
+    Duration? showDuration,
   }) {
     return TooltipThemeData(
       height: height ?? this.height,
@@ -119,7 +117,7 @@ class TooltipThemeData with Diagnosticable {
   /// If both arguments are null, then null is returned.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static TooltipThemeData lerp(TooltipThemeData a, TooltipThemeData b, double t) {
+  static TooltipThemeData? lerp(TooltipThemeData? a, TooltipThemeData? b, double t) {
     if (a == null && b == null)
       return null;
     assert(t != null);
@@ -128,8 +126,8 @@ class TooltipThemeData with Diagnosticable {
       padding: EdgeInsetsGeometry.lerp(a?.padding, b?.padding, t),
       margin: EdgeInsetsGeometry.lerp(a?.margin, b?.margin, t),
       verticalOffset: lerpDouble(a?.verticalOffset, b?.verticalOffset, t),
-      preferBelow: t < 0.5 ? a.preferBelow: b.preferBelow,
-      excludeFromSemantics: t < 0.5 ? a.excludeFromSemantics : b.excludeFromSemantics,
+      preferBelow: t < 0.5 ? a?.preferBelow: b?.preferBelow,
+      excludeFromSemantics: t < 0.5 ? a?.excludeFromSemantics : b?.excludeFromSemantics,
       decoration: Decoration.lerp(a?.decoration, b?.decoration, t),
       textStyle: TextStyle.lerp(a?.textStyle, b?.textStyle, t),
     );
@@ -222,9 +220,9 @@ class TooltipTheme extends InheritedTheme {
   ///
   /// The data argument must not be null.
   const TooltipTheme({
-    Key key,
-    @required this.data,
-    Widget child,
+    Key? key,
+    required this.data,
+    required Widget child,
   }) : assert(data != null), super(key: key, child: child);
 
   /// The properties for descendant [Tooltip] widgets.
@@ -240,13 +238,13 @@ class TooltipTheme extends InheritedTheme {
   /// TooltipThemeData theme = TooltipTheme.of(context);
   /// ```
   static TooltipThemeData of(BuildContext context) {
-    final TooltipTheme tooltipTheme = context.dependOnInheritedWidgetOfExactType<TooltipTheme>();
-    return tooltipTheme?.data ?? Theme.of(context).tooltipTheme;
+    final TooltipTheme? tooltipTheme = context.dependOnInheritedWidgetOfExactType<TooltipTheme>();
+    return tooltipTheme?.data ?? Theme.of(context)!.tooltipTheme;
   }
 
   @override
   Widget wrap(BuildContext context, Widget child) {
-    final TooltipTheme ancestorTheme = context.findAncestorWidgetOfExactType<TooltipTheme>();
+    final TooltipTheme? ancestorTheme = context.findAncestorWidgetOfExactType<TooltipTheme>();
     return identical(this, ancestorTheme) ? child : TooltipTheme(data: data, child: child);
   }