migrate some material files to nullsafety (#67078)

* migrate some material files to nullsafety

* review

packages/flutter/lib/src/material/app_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';
@@ -46,54 +44,54 @@ class AppBarTheme with Diagnosticable {
   /// Default value for [AppBar.brightness].
   ///
   /// If null, [AppBar] uses [ThemeData.primaryColorBrightness].
-  final Brightness brightness;
+  final Brightness? brightness;
 
   /// Default value for [AppBar.backgroundColor].
   ///
   /// If null, [AppBar] uses [ThemeData.primaryColor].
-  final Color color;
+  final Color? color;
 
   /// Default value for [AppBar.elevation].
   ///
   /// If null, [AppBar] uses a default value of 4.0.
-  final double elevation;
+  final double? elevation;
 
   /// Default value for [AppBar.shadowColor].
   ///
   /// If null, [AppBar] uses a default value of fully opaque black.
-  final Color shadowColor;
+  final Color? shadowColor;
 
   /// Default value for [AppBar.iconTheme].
   ///
   /// If null, [AppBar] uses [ThemeData.primaryIconTheme].
-  final IconThemeData iconTheme;
+  final IconThemeData? iconTheme;
 
   /// Default value for [AppBar.actionsIconTheme].
   ///
   /// If null, [AppBar] uses [ThemeData.primaryIconTheme].
-  final IconThemeData actionsIconTheme;
+  final IconThemeData? actionsIconTheme;
 
   /// Default value for [AppBar.textTheme].
   ///
   /// If null, [AppBar] uses [ThemeData.primaryTextTheme].
-  final TextTheme textTheme;
+  final TextTheme? textTheme;
 
   /// Default value for [AppBar.centerTitle].
   ///
   /// If null, the value is adapted to current [TargetPlatform].
-  final bool centerTitle;
+  final bool? centerTitle;
 
   /// Creates a copy of this object with the given fields replaced with the
   /// new values.
   AppBarTheme copyWith({
-    IconThemeData actionsIconTheme,
-    Brightness brightness,
-    Color color,
-    double elevation,
-    Color shadowColor,
-    IconThemeData iconTheme,
-    TextTheme textTheme,
-    bool centerTitle,
+    IconThemeData? actionsIconTheme,
+    Brightness? brightness,
+    Color? color,
+    double? elevation,
+    Color? shadowColor,
+    IconThemeData? iconTheme,
+    TextTheme? textTheme,
+    bool? centerTitle,
   }) {
     return AppBarTheme(
       brightness: brightness ?? this.brightness,
@@ -109,7 +107,7 @@ class AppBarTheme with Diagnosticable {
 
   /// The [ThemeData.appBarTheme] property of the ambient [Theme].
   static AppBarTheme of(BuildContext context) {
-    return Theme.of(context).appBarTheme;
+    return Theme.of(context)!.appBarTheme;
   }
 
   /// Linearly interpolate between two AppBar themes.
@@ -117,7 +115,7 @@ class AppBarTheme with Diagnosticable {
   /// The argument `t` must not be null.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static AppBarTheme lerp(AppBarTheme a, AppBarTheme b, double t) {
+  static AppBarTheme lerp(AppBarTheme? a, AppBarTheme? b, double t) {
     assert(t != null);
     return AppBarTheme(
       brightness: t < 0.5 ? a?.brightness : b?.brightness,

packages/flutter/lib/src/material/back_button.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/widgets.dart';
 
 import 'debug.dart';
@@ -27,7 +25,7 @@ import 'theme.dart';
 class BackButtonIcon extends StatelessWidget {
   /// Creates an icon that shows the appropriate "back" image for
   /// the current platform (as obtained from the [Theme]).
-  const BackButtonIcon({ Key key }) : super(key: key);
+  const BackButtonIcon({ Key? key }) : super(key: key);
 
   /// Returns the appropriate "back" icon for the given `platform`.
   static IconData _getIconData(TargetPlatform platform) {
@@ -41,12 +39,10 @@ class BackButtonIcon extends StatelessWidget {
       case TargetPlatform.macOS:
         return Icons.arrow_back_ios;
     }
-    assert(false);
-    return null;
   }
 
   @override
-  Widget build(BuildContext context) => Icon(_getIconData(Theme.of(context).platform));
+  Widget build(BuildContext context) => Icon(_getIconData(Theme.of(context)!.platform));
 }
 
 /// A material design back button.
@@ -78,13 +74,13 @@ class BackButtonIcon extends StatelessWidget {
 class BackButton extends StatelessWidget {
   /// Creates an [IconButton] with the appropriate "back" icon for the current
   /// target platform.
-  const BackButton({ Key key, this.color, this.onPressed }) : super(key: key);
+  const BackButton({ Key? key, this.color, this.onPressed }) : super(key: key);
 
   /// The color to use for the icon.
   ///
   /// Defaults to the [IconThemeData.color] specified in the ambient [IconTheme],
   /// which usually matches the ambient [Theme]'s [ThemeData.iconTheme].
-  final Color color;
+  final Color? color;
 
   /// An override callback to perform instead of the default behavior which is
   /// to pop the [Navigator].
@@ -94,7 +90,7 @@ class BackButton extends StatelessWidget {
   /// situations.
   ///
   /// Defaults to null.
-  final VoidCallback onPressed;
+  final VoidCallback? onPressed;
 
   @override
   Widget build(BuildContext context) {
@@ -102,10 +98,10 @@ class BackButton extends StatelessWidget {
     return IconButton(
       icon: const BackButtonIcon(),
       color: color,
-      tooltip: MaterialLocalizations.of(context).backButtonTooltip,
+      tooltip: MaterialLocalizations.of(context)!.backButtonTooltip,
       onPressed: () {
         if (onPressed != null) {
-          onPressed();
+          onPressed!();
         } else {
           Navigator.maybePop(context);
         }
@@ -132,13 +128,13 @@ class BackButton extends StatelessWidget {
 ///  * [IconButton], to create other material design icon buttons.
 class CloseButton extends StatelessWidget {
   /// Creates a Material Design close button.
-  const CloseButton({ Key key, this.color, this.onPressed }) : super(key: key);
+  const CloseButton({ Key? key, this.color, this.onPressed }) : super(key: key);
 
   /// The color to use for the icon.
   ///
   /// Defaults to the [IconThemeData.color] specified in the ambient [IconTheme],
   /// which usually matches the ambient [Theme]'s [ThemeData.iconTheme].
-  final Color color;
+  final Color? color;
 
   /// An override callback to perform instead of the default behavior which is
   /// to pop the [Navigator].
@@ -148,7 +144,7 @@ class CloseButton extends StatelessWidget {
   /// situations.
   ///
   /// Defaults to null.
-  final VoidCallback onPressed;
+  final VoidCallback? onPressed;
 
   @override
   Widget build(BuildContext context) {
@@ -156,10 +152,10 @@ class CloseButton extends StatelessWidget {
     return IconButton(
       icon: const Icon(Icons.close),
       color: color,
-      tooltip: MaterialLocalizations.of(context).closeButtonTooltip,
+      tooltip: MaterialLocalizations.of(context)!.closeButtonTooltip,
       onPressed: () {
         if (onPressed != null) {
-          onPressed();
+          onPressed!();
         } else {
           Navigator.maybePop(context);
         }

packages/flutter/lib/src/material/banner.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/widgets.dart';
 
 import 'banner_theme.dart';
@@ -35,10 +33,10 @@ class MaterialBanner extends StatelessWidget {
   /// The [actions], [content], and [forceActionsBelow] must be non-null.
   /// The [actions.length] must be greater than 0.
   const MaterialBanner({
-    Key key,
-    @required this.content,
+    Key? key,
+    required this.content,
     this.contentTextStyle,
-    @required this.actions,
+    required this.actions,
     this.leading,
     this.backgroundColor,
     this.padding,
@@ -58,7 +56,7 @@ class MaterialBanner extends StatelessWidget {
   ///
   /// If `null`, [MaterialBannerThemeData.contentTextStyle] is used. If that is
   /// also `null`, [TextTheme.bodyText2] of [ThemeData.textTheme] is used.
-  final TextStyle contentTextStyle;
+  final TextStyle? contentTextStyle;
 
   /// The set of actions that are displayed at the bottom or trailing side of
   /// the [MaterialBanner].
@@ -69,13 +67,13 @@ class MaterialBanner extends StatelessWidget {
   /// The (optional) leading widget of the [MaterialBanner].
   ///
   /// Typically an [Icon] widget.
-  final Widget leading;
+  final Widget? leading;
 
   /// The color of the surface of this [MaterialBanner].
   ///
   /// If `null`, [MaterialBannerThemeData.backgroundColor] is used. If that is
   /// also `null`, [ColorScheme.surface] of [ThemeData.colorScheme] is used.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// The amount of space by which to inset the [content].
   ///
@@ -84,12 +82,12 @@ class MaterialBanner extends StatelessWidget {
   ///
   /// If the [actions] are trailing the [content], this defaults to
   /// `EdgeInsetsDirectional.only(start: 16.0, top: 2.0)`.
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   /// The amount of space by which to inset the [leading] widget.
   ///
   /// This defaults to `EdgeInsetsDirectional.only(end: 16.0)`.
-  final EdgeInsetsGeometry leadingPadding;
+  final EdgeInsetsGeometry? leadingPadding;
 
   /// An override to force the [actions] to be below the [content] regardless of
   /// how many there are.
@@ -104,7 +102,7 @@ class MaterialBanner extends StatelessWidget {
   Widget build(BuildContext context) {
     assert(actions.isNotEmpty);
 
-    final ThemeData theme = Theme.of(context);
+    final ThemeData? theme = Theme.of(context);
     final MaterialBannerThemeData bannerTheme = MaterialBannerTheme.of(context);
 
     final bool isSingleRow = actions.length == 1 && !forceActionsBelow;
@@ -127,10 +125,10 @@ class MaterialBanner extends StatelessWidget {
 
     final Color backgroundColor = this.backgroundColor
         ?? bannerTheme.backgroundColor
-        ?? theme.colorScheme.surface;
-    final TextStyle textStyle = contentTextStyle
+        ?? theme!.colorScheme.surface;
+    final TextStyle? textStyle = contentTextStyle
         ?? bannerTheme.contentTextStyle
-        ?? theme.textTheme.bodyText2;
+        ?? theme!.textTheme.bodyText2;
 
     return Container(
       color: backgroundColor,

packages/flutter/lib/src/material/banner_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/foundation.dart';
 import 'package:flutter/widgets.dart';
 
@@ -38,25 +36,25 @@ class MaterialBannerThemeData with Diagnosticable {
   });
 
   /// The background color of a [MaterialBanner].
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// Used to configure the [DefaultTextStyle] for the [MaterialBanner.content]
   /// widget.
-  final TextStyle contentTextStyle;
+  final TextStyle? contentTextStyle;
 
   /// The amount of space by which to inset [MaterialBanner.content].
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   /// The amount of space by which to inset [MaterialBanner.leading].
-  final EdgeInsetsGeometry leadingPadding;
+  final EdgeInsetsGeometry? leadingPadding;
 
   /// Creates a copy of this object with the given fields replaced with the
   /// new values.
   MaterialBannerThemeData copyWith({
-    Color backgroundColor,
-    TextStyle contentTextStyle,
-    EdgeInsetsGeometry padding,
-    EdgeInsetsGeometry leadingPadding,
+    Color? backgroundColor,
+    TextStyle? contentTextStyle,
+    EdgeInsetsGeometry? padding,
+    EdgeInsetsGeometry? leadingPadding,
   }) {
     return MaterialBannerThemeData(
       backgroundColor: backgroundColor ?? this.backgroundColor,
@@ -71,7 +69,7 @@ class MaterialBannerThemeData with Diagnosticable {
   /// The argument `t` must not be null.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static MaterialBannerThemeData lerp(MaterialBannerThemeData a, MaterialBannerThemeData b, double t) {
+  static MaterialBannerThemeData lerp(MaterialBannerThemeData? a, MaterialBannerThemeData? b, double t) {
     assert(t != null);
     return MaterialBannerThemeData(
       backgroundColor: Color.lerp(a?.backgroundColor, b?.backgroundColor, t),
@@ -123,13 +121,13 @@ class MaterialBannerTheme extends InheritedTheme {
   /// Creates a banner theme that controls the configurations for
   /// [MaterialBanner]s in its widget subtree.
   const MaterialBannerTheme({
-    Key key,
+    Key? key,
     this.data,
-    Widget child,
+    required Widget child,
   }) : super(key: key, child: child);
 
   /// The properties for descendant [MaterialBanner] widgets.
-  final MaterialBannerThemeData data;
+  final MaterialBannerThemeData? data;
 
   /// The closest instance of this class's [data] value that encloses the given
   /// context.
@@ -143,13 +141,13 @@ class MaterialBannerTheme extends InheritedTheme {
   /// MaterialBannerThemeData theme = MaterialBannerTheme.of(context);
   /// ```
   static MaterialBannerThemeData of(BuildContext context) {
-    final MaterialBannerTheme bannerTheme = context.dependOnInheritedWidgetOfExactType<MaterialBannerTheme>();
-    return bannerTheme?.data ?? Theme.of(context).bannerTheme;
+    final MaterialBannerTheme? bannerTheme = context.dependOnInheritedWidgetOfExactType<MaterialBannerTheme>();
+    return bannerTheme?.data ?? Theme.of(context)!.bannerTheme;
   }
 
   @override
   Widget wrap(BuildContext context, Widget child) {
-    final MaterialBannerTheme ancestorTheme = context.findAncestorWidgetOfExactType<MaterialBannerTheme>();
+    final MaterialBannerTheme? ancestorTheme = context.findAncestorWidgetOfExactType<MaterialBannerTheme>();
     return identical(this, ancestorTheme) ? child : MaterialBannerTheme(data: data, child: child);
   }
 

packages/flutter/lib/src/material/bottom_app_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/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -50,7 +48,7 @@ class BottomAppBar extends StatefulWidget {
   /// If the corresponding [BottomAppBarTheme] property is null, then the default
   /// specified in the property's documentation will be used.
   const BottomAppBar({
-    Key key,
+    Key? key,
     this.color,
     this.elevation,
     this.shape,
@@ -68,14 +66,14 @@ class BottomAppBar extends StatefulWidget {
   ///
   /// Typically this the child will be a [Row], with the first child
   /// being an [IconButton] with the [Icons.menu] icon.
-  final Widget child;
+  final Widget? child;
 
   /// The bottom app bar's background color.
   ///
   /// If this property is null then [BottomAppBarTheme.color] of
   /// [ThemeData.bottomAppBarTheme] is used. If that's null then
   /// [ThemeData.bottomAppBarColor] is used.
-  final Color color;
+  final Color? color;
 
   /// The z-coordinate at which to place this bottom app bar relative to its
   /// parent.
@@ -86,14 +84,14 @@ class BottomAppBar extends StatefulWidget {
   /// If this property is null then [BottomAppBarTheme.elevation] of
   /// [ThemeData.bottomAppBarTheme] is used. If that's null, the default value
   /// is 8.
-  final double elevation;
+  final double? elevation;
 
   /// The notch that is made for the floating action button.
   ///
   /// If this property is null then [BottomAppBarTheme.shape] of
   /// [ThemeData.bottomAppBarTheme] is used. If that's null then the shape will
   /// be rectangular with no notch.
-  final NotchedShape shape;
+  final NotchedShape? shape;
 
   /// {@macro flutter.widgets.Clip}
   ///
@@ -111,19 +109,19 @@ class BottomAppBar extends StatefulWidget {
 }
 
 class _BottomAppBarState extends State<BottomAppBar> {
-  ValueListenable<ScaffoldGeometry> geometryListenable;
+  late ValueListenable<ScaffoldGeometry> geometryListenable;
   static const double _defaultElevation = 8.0;
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    geometryListenable = Scaffold.geometryOf(context);
+    geometryListenable = Scaffold.geometryOf(context)!;
   }
 
   @override
   Widget build(BuildContext context) {
     final BottomAppBarTheme babTheme = BottomAppBarTheme.of(context);
-    final NotchedShape notchedShape = widget.shape ?? babTheme.shape;
+    final NotchedShape? notchedShape = widget.shape ?? babTheme.shape;
     final CustomClipper<Path> clipper = notchedShape != null
       ? _BottomAppBarClipper(
         geometry: geometryListenable,
@@ -132,7 +130,7 @@ class _BottomAppBarState extends State<BottomAppBar> {
       )
       : const ShapeBorderClipper(shape: RoundedRectangleBorder());
     final double elevation = widget.elevation ?? babTheme.elevation ?? _defaultElevation;
-    final Color color = widget.color ?? babTheme.color ?? Theme.of(context).bottomAppBarColor;
+    final Color color = widget.color ?? babTheme.color ?? Theme.of(context)!.bottomAppBarColor;
     final Color effectiveColor = ElevationOverlay.applyOverlay(context, color, elevation);
     return PhysicalShape(
       clipper: clipper,
@@ -143,7 +141,7 @@ class _BottomAppBarState extends State<BottomAppBar> {
         type: MaterialType.transparency,
         child: widget.child == null
           ? null
-          : SafeArea(child: widget.child),
+          : SafeArea(child: widget.child!),
       ),
     );
   }
@@ -151,9 +149,9 @@ class _BottomAppBarState extends State<BottomAppBar> {
 
 class _BottomAppBarClipper extends CustomClipper<Path> {
   const _BottomAppBarClipper({
-    @required this.geometry,
-    @required this.shape,
-    @required this.notchMargin,
+    required this.geometry,
+    required this.shape,
+    required this.notchMargin,
   }) : assert(geometry != null),
        assert(shape != null),
        assert(notchMargin != null),
@@ -168,9 +166,9 @@ class _BottomAppBarClipper extends CustomClipper<Path> {
     // button is the floating action button's bounding rectangle in the
     // coordinate system whose origin is at the appBar's top left corner,
     // or null if there is no floating action button.
-    final Rect button = geometry.value.floatingActionButtonArea?.translate(
+    final Rect? button = geometry.value.floatingActionButtonArea?.translate(
       0.0,
-      geometry.value.bottomNavigationBarTop * -1.0,
+      geometry.value.bottomNavigationBarTop! * -1.0,
     );
     return shape.getOuterPath(Offset.zero & size, button?.inflate(notchMargin));
   }

packages/flutter/lib/src/material/bottom_app_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';
@@ -39,20 +37,20 @@ class BottomAppBarTheme with Diagnosticable {
   /// Default value for [BottomAppBar.color].
   ///
   /// If null, [BottomAppBar] uses [ThemeData.bottomAppBarColor].
-  final Color color;
+  final Color? color;
 
   /// Default value for [BottomAppBar.elevation].
-  final double elevation;
+  final double? elevation;
 
   /// Default value for [BottomAppBar.shape].
-  final NotchedShape shape;
+  final NotchedShape? shape;
 
   /// Creates a copy of this object but with the given fields replaced with the
   /// new values.
   BottomAppBarTheme copyWith({
-    Color color,
-    double elevation,
-    NotchedShape shape,
+    Color? color,
+    double? elevation,
+    NotchedShape? shape,
   }) {
     return BottomAppBarTheme(
       color: color ?? this.color,
@@ -63,7 +61,7 @@ class BottomAppBarTheme with Diagnosticable {
 
   /// The [ThemeData.bottomAppBarTheme] property of the ambient [Theme].
   static BottomAppBarTheme of(BuildContext context) {
-    return Theme.of(context).bottomAppBarTheme;
+    return Theme.of(context)!.bottomAppBarTheme;
   }
 
   /// Linearly interpolate between two BAB themes.
@@ -71,7 +69,7 @@ class BottomAppBarTheme with Diagnosticable {
   /// The argument `t` must not be null.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static BottomAppBarTheme lerp(BottomAppBarTheme a, BottomAppBarTheme b, double t) {
+  static BottomAppBarTheme lerp(BottomAppBarTheme? a, BottomAppBarTheme? b, double t) {
     assert(t != null);
     return BottomAppBarTheme(
       color: Color.lerp(a?.color, b?.color, t),

packages/flutter/lib/src/material/bottom_navigation_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';
@@ -50,78 +48,78 @@ class BottomNavigationBarThemeData with Diagnosticable {
   /// The color of the [BottomNavigationBar] itself.
   ///
   /// See [BottomNavigationBar.backgroundColor].
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// The z-coordinate of the [BottomNavigationBar].
   ///
   /// See [BottomNavigationBar.elevation].
-  final double elevation;
+  final double? elevation;
 
   /// The size, opacity, and color of the icon in the currently selected
   /// [BottomNavigationBarItem.icon].
   ///
   /// See [BottomNavigationBar.selectedIconTheme].
-  final IconThemeData selectedIconTheme;
+  final IconThemeData? selectedIconTheme;
 
   /// The size, opacity, and color of the icon in the currently unselected
   /// [BottomNavigationBarItem.icon]s.
   ///
   /// See [BottomNavigationBar.unselectedIconTheme].
-  final IconThemeData unselectedIconTheme;
+  final IconThemeData? unselectedIconTheme;
 
   /// The color of the selected [BottomNavigationBarItem.icon] and
   /// [BottomNavigationBarItem.title].
   ///
   /// See [BottomNavigationBar.selectedItemColor].
-  final Color selectedItemColor;
+  final Color? selectedItemColor;
 
   /// The color of the unselected [BottomNavigationBarItem.icon] and
   /// [BottomNavigationBarItem.title]s.
   ///
   /// See [BottomNavigationBar.unselectedItemColor].
-  final Color unselectedItemColor;
+  final Color? unselectedItemColor;
 
   /// The [TextStyle] of the [BottomNavigationBarItem] labels when they are
   /// selected.
   ///
   /// See [BottomNavigationBar.selectedLabelStyle].
-  final TextStyle selectedLabelStyle;
+  final TextStyle? selectedLabelStyle;
 
   /// The [TextStyle] of the [BottomNavigationBarItem] labels when they are not
   /// selected.
   ///
   /// See [BottomNavigationBar.unselectedLabelStyle].
-  final TextStyle unselectedLabelStyle;
+  final TextStyle? unselectedLabelStyle;
 
   /// Whether the labels are shown for the unselected [BottomNavigationBarItem]s.
   ///
   /// See [BottomNavigationBar.showSelectedLabels].
-  final bool showSelectedLabels;
+  final bool? showSelectedLabels;
 
   /// Whether the labels are shown for the selected [BottomNavigationBarItem].
   ///
   /// See [BottomNavigationBar.showUnselectedLabels].
-  final bool showUnselectedLabels;
+  final bool? showUnselectedLabels;
 
   /// Defines the layout and behavior of a [BottomNavigationBar].
   ///
   /// See [BottomNavigationBar.type].
-  final BottomNavigationBarType type;
+  final BottomNavigationBarType? type;
 
   /// Creates a copy of this object but with the given fields replaced with the
   /// new values.
   BottomNavigationBarThemeData copyWith({
-    Color backgroundColor,
-    double elevation,
-    IconThemeData selectedIconTheme,
-    IconThemeData unselectedIconTheme,
-    Color selectedItemColor,
-    Color unselectedItemColor,
-    TextStyle selectedLabelStyle,
-    TextStyle unselectedLabelStyle,
-    bool showSelectedLabels,
-    bool showUnselectedLabels,
-    BottomNavigationBarType type,
+    Color? backgroundColor,
+    double? elevation,
+    IconThemeData? selectedIconTheme,
+    IconThemeData? unselectedIconTheme,
+    Color? selectedItemColor,
+    Color? unselectedItemColor,
+    TextStyle? selectedLabelStyle,
+    TextStyle? unselectedLabelStyle,
+    bool? showSelectedLabels,
+    bool? showUnselectedLabels,
+    BottomNavigationBarType? type,
   }) {
     return BottomNavigationBarThemeData(
       backgroundColor: backgroundColor ?? this.backgroundColor,
@@ -143,7 +141,7 @@ class BottomNavigationBarThemeData with Diagnosticable {
   /// The argument `t` must not be null.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static BottomNavigationBarThemeData lerp(BottomNavigationBarThemeData a, BottomNavigationBarThemeData b, double t) {
+  static BottomNavigationBarThemeData lerp(BottomNavigationBarThemeData? a, BottomNavigationBarThemeData? b, double t) {
     assert(t != null);
     return BottomNavigationBarThemeData(
       backgroundColor: Color.lerp(a?.backgroundColor, b?.backgroundColor, t),
@@ -235,9 +233,9 @@ class BottomNavigationBarTheme extends InheritedWidget {
   ///
   /// The [data] must not be null.
   const BottomNavigationBarTheme({
-    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 used for all descendant [BottomNavigationBar] widgets.
@@ -254,8 +252,8 @@ class BottomNavigationBarTheme extends InheritedWidget {
   /// BottomNavigationBarThemeData theme = BottomNavigationBarTheme.of(context);
   /// ```
   static BottomNavigationBarThemeData of(BuildContext context) {
-    final BottomNavigationBarTheme bottomNavTheme = context.dependOnInheritedWidgetOfExactType<BottomNavigationBarTheme>();
-    return bottomNavTheme?.data ?? Theme.of(context).bottomNavigationBarTheme;
+    final BottomNavigationBarTheme? bottomNavTheme = context.dependOnInheritedWidgetOfExactType<BottomNavigationBarTheme>();
+    return bottomNavTheme?.data ?? Theme.of(context)!.bottomNavigationBarTheme;
   }
 
   @override

packages/flutter/lib/src/material/button.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/foundation.dart';
@@ -51,8 +49,8 @@ class RawMaterialButton extends StatefulWidget {
   /// [elevation], [focusElevation], [hoverElevation], [highlightElevation], and
   /// [disabledElevation] must be non-negative.
   const RawMaterialButton({
-    Key key,
-    @required this.onPressed,
+    Key? key,
+    required this.onPressed,
     this.onLongPress,
     this.onHighlightChanged,
     this.mouseCursor,
@@ -75,7 +73,7 @@ class RawMaterialButton extends StatefulWidget {
     this.clipBehavior = Clip.none,
     this.focusNode,
     this.autofocus = false,
-    MaterialTapTargetSize materialTapTargetSize,
+    MaterialTapTargetSize? materialTapTargetSize,
     this.child,
     this.enableFeedback = true,
   }) : materialTapTargetSize = materialTapTargetSize ?? MaterialTapTargetSize.padded,
@@ -99,7 +97,7 @@ class RawMaterialButton extends StatefulWidget {
   /// See also:
   ///
   ///  * [enabled], which is true if the button is enabled.
-  final VoidCallback onPressed;
+  final VoidCallback? onPressed;
 
   /// Called when the button is long-pressed.
   ///
@@ -108,7 +106,7 @@ class RawMaterialButton extends StatefulWidget {
   /// See also:
   ///
   ///  * [enabled], which is true if the button is enabled.
-  final VoidCallback onLongPress;
+  final VoidCallback? onLongPress;
 
   /// Called by the underlying [InkWell] widget's [InkWell.onHighlightChanged]
   /// callback.
@@ -116,7 +114,7 @@ class RawMaterialButton extends StatefulWidget {
   /// If [onPressed] changes from null to non-null while a gesture is ongoing,
   /// this can fire during the build phase (in which case calling
   /// [State.setState] is not allowed).
-  final ValueChanged<bool> onHighlightChanged;
+  final ValueChanged<bool>? onHighlightChanged;
 
   /// {@template flutter.material.button.mouseCursor}
   /// The cursor for a mouse pointer when it enters or is hovering over the
@@ -132,7 +130,7 @@ class RawMaterialButton extends StatefulWidget {
   ///
   /// If this property is null, [MaterialStateMouseCursor.clickable] will be used.
   /// {@endtemplate}
-  final MouseCursor mouseCursor;
+  final MouseCursor? mouseCursor;
 
   /// Defines the default text style, with [Material.textStyle], for the
   /// button's [child].
@@ -144,22 +142,22 @@ class RawMaterialButton extends StatefulWidget {
   ///  * [MaterialState.hovered].
   ///  * [MaterialState.focused].
   ///  * [MaterialState.disabled].
-  final TextStyle textStyle;
+  final TextStyle? textStyle;
 
   /// The color of the button's [Material].
-  final Color fillColor;
+  final Color? fillColor;
 
   /// The color for the button's [Material] when it has the input focus.
-  final Color focusColor;
+  final Color? focusColor;
 
   /// The color for the button's [Material] when a pointer is hovering over it.
-  final Color hoverColor;
+  final Color? hoverColor;
 
   /// The highlight color for the button's [InkWell].
-  final Color highlightColor;
+  final Color? highlightColor;
 
   /// The splash color for the button's [InkWell].
-  final Color splashColor;
+  final Color? splashColor;
 
   /// The elevation for the button's [Material] when the button
   /// is [enabled] but not pressed.
@@ -275,7 +273,7 @@ class RawMaterialButton extends StatefulWidget {
   final Duration animationDuration;
 
   /// Typically the button's label.
-  final Widget child;
+  final Widget? child;
 
   /// Whether the button is enabled or disabled.
   ///
@@ -293,7 +291,7 @@ class RawMaterialButton extends StatefulWidget {
   final MaterialTapTargetSize materialTapTargetSize;
 
   /// {@macro flutter.widgets.Focus.focusNode}
-  final FocusNode focusNode;
+  final FocusNode? focusNode;
 
   /// {@macro flutter.widgets.Focus.autofocus}
   final bool autofocus;
@@ -334,7 +332,7 @@ class _RawMaterialButtonState extends State<RawMaterialButton> {
       setState(() {
         _updateState(MaterialState.pressed, value);
         if (widget.onHighlightChanged != null) {
-          widget.onHighlightChanged(value);
+          widget.onHighlightChanged!(value);
         }
       });
     }
@@ -395,11 +393,11 @@ class _RawMaterialButtonState extends State<RawMaterialButton> {
 
   @override
   Widget build(BuildContext context) {
-    final Color effectiveTextColor = MaterialStateProperty.resolveAs<Color>(widget.textStyle?.color, _states);
-    final ShapeBorder effectiveShape =  MaterialStateProperty.resolveAs<ShapeBorder>(widget.shape, _states);
+    final Color? effectiveTextColor = MaterialStateProperty.resolveAs<Color>(widget.textStyle?.color, _states);
+    final ShapeBorder? effectiveShape =  MaterialStateProperty.resolveAs<ShapeBorder>(widget.shape, _states);
     final Offset densityAdjustment = widget.visualDensity.baseSizeAdjustment;
     final BoxConstraints effectiveConstraints = widget.visualDensity.effectiveConstraints(widget.constraints);
-    final MouseCursor effectiveMouseCursor = MaterialStateProperty.resolveAs<MouseCursor>(
+    final MouseCursor? effectiveMouseCursor = MaterialStateProperty.resolveAs<MouseCursor>(
       widget.mouseCursor ?? MaterialStateMouseCursor.clickable,
       _states,
     );
@@ -487,9 +485,9 @@ class _RawMaterialButtonState extends State<RawMaterialButton> {
 /// "tap target", but not its material or its ink splashes.
 class _InputPadding extends SingleChildRenderObjectWidget {
   const _InputPadding({
-    Key key,
-    Widget child,
-    this.minSize,
+    Key? key,
+    Widget? child,
+    required this.minSize,
   }) : super(key: key, child: child);
 
   final Size minSize;
@@ -506,7 +504,7 @@ class _InputPadding extends SingleChildRenderObjectWidget {
 }
 
 class _RenderInputPadding extends RenderShiftedBox {
-  _RenderInputPadding(this._minSize, [RenderBox child]) : super(child);
+  _RenderInputPadding(this._minSize, [RenderBox? child]) : super(child);
 
   Size get minSize => _minSize;
   Size _minSize;
@@ -520,58 +518,57 @@ class _RenderInputPadding extends RenderShiftedBox {
   @override
   double computeMinIntrinsicWidth(double height) {
     if (child != null)
-      return math.max(child.getMinIntrinsicWidth(height), minSize.width);
+      return math.max(child!.getMinIntrinsicWidth(height), minSize.width);
     return 0.0;
   }
 
   @override
   double computeMinIntrinsicHeight(double width) {
     if (child != null)
-      return math.max(child.getMinIntrinsicHeight(width), minSize.height);
+      return math.max(child!.getMinIntrinsicHeight(width), minSize.height);
     return 0.0;
   }
 
   @override
   double computeMaxIntrinsicWidth(double height) {
     if (child != null)
-      return math.max(child.getMaxIntrinsicWidth(height), minSize.width);
+      return math.max(child!.getMaxIntrinsicWidth(height), minSize.width);
     return 0.0;
   }
 
   @override
   double computeMaxIntrinsicHeight(double width) {
     if (child != null)
-      return math.max(child.getMaxIntrinsicHeight(width), minSize.height);
+      return math.max(child!.getMaxIntrinsicHeight(width), minSize.height);
     return 0.0;
   }
 
   @override
   void performLayout() {
-    final BoxConstraints constraints = this.constraints;
     if (child != null) {
-      child.layout(constraints, parentUsesSize: true);
-      final double height = math.max(child.size.width, minSize.width);
-      final double width = math.max(child.size.height, minSize.height);
+      child!.layout(constraints, parentUsesSize: true);
+      final double height = math.max(child!.size.width, minSize.width);
+      final double width = math.max(child!.size.height, minSize.height);
       size = constraints.constrain(Size(height, width));
-      final BoxParentData childParentData = child.parentData as BoxParentData;
-      childParentData.offset = Alignment.center.alongOffset(size - child.size as Offset);
+      final BoxParentData childParentData = child!.parentData as BoxParentData;
+      childParentData.offset = Alignment.center.alongOffset(size - child!.size as Offset);
     } else {
       size = Size.zero;
     }
   }
 
   @override
-  bool hitTest(BoxHitTestResult result, { Offset position }) {
+  bool hitTest(BoxHitTestResult result, { required Offset position }) {
     if (super.hitTest(result, position: position)) {
       return true;
     }
-    final Offset center = child.size.center(Offset.zero);
+    final Offset center = child!.size.center(Offset.zero);
     return result.addWithRawTransform(
       transform: MatrixUtils.forceToPoint(center),
       position: center,
-      hitTest: (BoxHitTestResult result, Offset position) {
+      hitTest: (BoxHitTestResult result, Offset? position) {
         assert(position == center);
-        return child.hitTest(result, position: center);
+        return child!.hitTest(result, position: center);
       },
     );
   }

packages/flutter/lib/src/material/button_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/widgets.dart';
 import 'package:flutter/rendering.dart';
 
@@ -56,7 +54,7 @@ class ButtonBar extends StatelessWidget {
   /// Both [buttonMinWidth] and [buttonHeight] must be non-negative if they
   /// are not null.
   const ButtonBar({
-    Key key,
+    Key? key,
     this.alignment,
     this.mainAxisSize,
     this.buttonTextTheme,
@@ -77,13 +75,13 @@ class ButtonBar extends StatelessWidget {
   ///
   /// If null then it will use [ButtonBarThemeData.alignment]. If that is null,
   /// it will default to [MainAxisAlignment.end].
-  final MainAxisAlignment alignment;
+  final MainAxisAlignment? alignment;
 
   /// How much horizontal space is available. See [Row.mainAxisSize].
   ///
   /// If null then it will use the surrounding [ButtonBarThemeData.mainAxisSize].
   /// If that is null, it will default to [MainAxisSize.max].
-  final MainAxisSize mainAxisSize;
+  final MainAxisSize? mainAxisSize;
 
   /// Overrides the surrounding [ButtonBarThemeData.buttonTextTheme] to define a
   /// button's base colors, size, internal padding and shape.
@@ -91,21 +89,21 @@ class ButtonBar extends StatelessWidget {
   /// If null then it will use the surrounding
   /// [ButtonBarThemeData.buttonTextTheme]. If that is null, it will default to
   /// [ButtonTextTheme.primary].
-  final ButtonTextTheme buttonTextTheme;
+  final ButtonTextTheme? buttonTextTheme;
 
   /// Overrides the surrounding [ButtonThemeData.minWidth] to define a button's
   /// minimum width.
   ///
   /// If null then it will use the surrounding [ButtonBarThemeData.buttonMinWidth].
   /// If that is null, it will default to 64.0 logical pixels.
-  final double buttonMinWidth;
+  final double? buttonMinWidth;
 
   /// Overrides the surrounding [ButtonThemeData.height] to define a button's
   /// minimum height.
   ///
   /// If null then it will use the surrounding [ButtonBarThemeData.buttonHeight].
   /// If that is null, it will default to 36.0 logical pixels.
-  final double buttonHeight;
+  final double? buttonHeight;
 
   /// Overrides the surrounding [ButtonThemeData.padding] to define the padding
   /// for a button's child (typically the button's label).
@@ -113,14 +111,14 @@ class ButtonBar extends StatelessWidget {
   /// If null then it will use the surrounding [ButtonBarThemeData.buttonPadding].
   /// If that is null, it will default to 8.0 logical pixels on the left
   /// and right.
-  final EdgeInsetsGeometry buttonPadding;
+  final EdgeInsetsGeometry? buttonPadding;
 
   /// Overrides the surrounding [ButtonThemeData.alignedDropdown] to define whether
   /// a [DropdownButton] menu's width will match the button's width.
   ///
   /// If null then it will use the surrounding [ButtonBarThemeData.buttonAlignedDropdown].
   /// If that is null, it will default to false.
-  final bool buttonAlignedDropdown;
+  final bool? buttonAlignedDropdown;
 
   /// Defines whether a [ButtonBar] should size itself with a minimum size
   /// constraint or with padding.
@@ -129,7 +127,7 @@ class ButtonBar extends StatelessWidget {
   ///
   /// If null then it will use the surrounding [ButtonBarThemeData.layoutBehavior].
   /// If that is null, it will default [ButtonBarLayoutBehavior.padded].
-  final ButtonBarLayoutBehavior layoutBehavior;
+  final ButtonBarLayoutBehavior? layoutBehavior;
 
   /// Defines the vertical direction of a [ButtonBar]'s children if it
   /// overflows.
@@ -145,7 +143,7 @@ class ButtonBar extends StatelessWidget {
   /// If null then it will use the surrounding
   /// [ButtonBarThemeData.overflowDirection]. If that is null, it will
   /// default to [VerticalDirection.down].
-  final VerticalDirection overflowDirection;
+  final VerticalDirection? overflowDirection;
 
   /// The spacing between buttons when the button bar overflows.
   ///
@@ -161,7 +159,7 @@ class ButtonBar extends StatelessWidget {
   ///
   /// If null then no spacing will be added in between buttons in
   /// an overflow state.
-  final double overflowButtonSpacing;
+  final double? overflowButtonSpacing;
 
   /// The buttons to arrange horizontally.
   ///
@@ -216,8 +214,6 @@ class ButtonBar extends StatelessWidget {
           child: child,
         );
     }
-    assert(false);
-    return null;
   }
 }
 
@@ -239,14 +235,14 @@ class _ButtonBarRow extends Flex {
   /// Creates a button bar that attempts to display in a row, but displays in
   /// a column if there is insufficient horizontal space.
   _ButtonBarRow({
-    List<Widget> children,
+    required List<Widget> children,
     Axis direction = Axis.horizontal,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection overflowDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
+    TextBaseline? textBaseline,
     this.overflowButtonSpacing,
   }) : super(
     children: children,
@@ -259,7 +255,7 @@ class _ButtonBarRow extends Flex {
     textBaseline: textBaseline,
   );
 
-  final double overflowButtonSpacing;
+  final double? overflowButtonSpacing;
 
   @override
   _RenderButtonBarRow createRenderObject(BuildContext context) {
@@ -268,7 +264,7 @@ class _ButtonBarRow extends Flex {
       mainAxisAlignment: mainAxisAlignment,
       mainAxisSize: mainAxisSize,
       crossAxisAlignment: crossAxisAlignment,
-      textDirection: getEffectiveTextDirection(context),
+      textDirection: getEffectiveTextDirection(context)!,
       verticalDirection: verticalDirection,
       textBaseline: textBaseline,
       overflowButtonSpacing: overflowButtonSpacing,
@@ -307,14 +303,14 @@ class _RenderButtonBarRow extends RenderFlex {
   /// Creates a button bar that attempts to display in a row, but displays in
   /// a column if there is insufficient horizontal space.
   _RenderButtonBarRow({
-    List<RenderBox> children,
+    List<RenderBox>? children,
     Axis direction = Axis.horizontal,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    @required TextDirection textDirection,
+    required TextDirection textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
+    TextBaseline? textBaseline,
     this.overflowButtonSpacing,
   }) : assert(textDirection != null),
        assert(overflowButtonSpacing == null || overflowButtonSpacing >= 0),
@@ -330,13 +326,13 @@ class _RenderButtonBarRow extends RenderFlex {
        );
 
   bool _hasCheckedLayoutWidth = false;
-  double overflowButtonSpacing;
+  double? overflowButtonSpacing;
 
   @override
   BoxConstraints get constraints {
     if (_hasCheckedLayoutWidth)
       return super.constraints;
-    return super.constraints?.copyWith(maxWidth: double.infinity);
+    return super.constraints.copyWith(maxWidth: double.infinity);
   }
 
   @override
@@ -359,7 +355,7 @@ class _RenderButtonBarRow extends RenderFlex {
       super.performLayout();
     } else {
       final BoxConstraints childConstraints = constraints.copyWith(minWidth: 0.0);
-      RenderBox child;
+      RenderBox? child;
       double currentHeight = 0.0;
       switch (verticalDirection) {
         case VerticalDirection.down:
@@ -381,7 +377,7 @@ class _RenderButtonBarRow extends RenderFlex {
         // alignment for a row. For [MainAxisAlignment.spaceAround],
         // [MainAxisAlignment.spaceBetween] and [MainAxisAlignment.spaceEvenly]
         // cases, use [MainAxisAlignment.start].
-        switch (textDirection) {
+        switch (textDirection!) {
           case TextDirection.ltr:
             switch (mainAxisAlignment) {
               case MainAxisAlignment.center:
@@ -422,7 +418,7 @@ class _RenderButtonBarRow extends RenderFlex {
         }
 
         if (overflowButtonSpacing != null && child != null)
-          currentHeight += overflowButtonSpacing;
+          currentHeight += overflowButtonSpacing!;
       }
       size = constraints.constrain(Size(constraints.maxWidth, currentHeight));
     }

packages/flutter/lib/src/material/button_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';
@@ -46,10 +44,10 @@ class ButtonBarThemeData with Diagnosticable {
        assert(buttonHeight == null || buttonHeight >= 0.0);
 
   /// How the children should be placed along the horizontal axis.
-  final MainAxisAlignment alignment;
+  final MainAxisAlignment? alignment;
 
   /// How much horizontal space is available. See [Row.mainAxisSize].
-  final MainAxisSize mainAxisSize;
+  final MainAxisSize? mainAxisSize;
 
   /// Defines a [ButtonBar] button's base colors, and the defaults for
   /// the button's minimum size, internal padding, and shape.
@@ -59,7 +57,7 @@ class ButtonBarThemeData with Diagnosticable {
   ///
   /// Despite the name, this property is not a [TextTheme], its value is not a
   /// collection of [TextStyle]s.
-  final ButtonTextTheme buttonTextTheme;
+  final ButtonTextTheme? buttonTextTheme;
 
   /// The minimum width for [ButtonBar] buttons.
   ///
@@ -68,19 +66,19 @@ class ButtonBarThemeData with Diagnosticable {
   ///
   /// The actual horizontal space allocated for a button's child is
   /// at least this value less the theme's horizontal [ButtonThemeData.padding].
-  final double buttonMinWidth;
+  final double? buttonMinWidth;
 
   /// The minimum height for [ButtonBar] buttons.
   ///
   /// This will override the surrounding [ButtonThemeData.height] setting
   /// for buttons contained in the [ButtonBar].
-  final double buttonHeight;
+  final double? buttonHeight;
 
   /// Padding for a [ButtonBar] button's child (typically the button's label).
   ///
   /// This will override the surrounding [ButtonThemeData.padding] setting
   /// for buttons contained in the [ButtonBar].
-  final EdgeInsetsGeometry buttonPadding;
+  final EdgeInsetsGeometry? buttonPadding;
 
   /// If true, then a [DropdownButton] menu's width will match the [ButtonBar]
   /// button's width.
@@ -95,11 +93,11 @@ class ButtonBarThemeData with Diagnosticable {
   ///
   /// This property only affects [DropdownButton] contained in a [ButtonBar]
   /// and its menu.
-  final bool buttonAlignedDropdown;
+  final bool? buttonAlignedDropdown;
 
   /// Defines whether a [ButtonBar] should size itself with a minimum size
   /// constraint or with padding.
-  final ButtonBarLayoutBehavior layoutBehavior;
+  final ButtonBarLayoutBehavior? layoutBehavior;
 
   /// Defines the vertical direction of a [ButtonBar]'s children if it
   /// overflows.
@@ -111,20 +109,20 @@ class ButtonBarThemeData with Diagnosticable {
   /// the first action will be at the bottom of the column if this
   /// property is set to [VerticalDirection.up], since it "starts" at the
   /// bottom and "ends" at the top.
-  final VerticalDirection overflowDirection;
+  final VerticalDirection? overflowDirection;
 
   /// Creates a copy of this object but with the given fields replaced with the
   /// new values.
   ButtonBarThemeData copyWith({
-    MainAxisAlignment alignment,
-    MainAxisSize mainAxisSize,
-    ButtonTextTheme buttonTextTheme,
-    double buttonMinWidth,
-    double buttonHeight,
-    EdgeInsetsGeometry buttonPadding,
-    bool buttonAlignedDropdown,
-    ButtonBarLayoutBehavior layoutBehavior,
-    VerticalDirection overflowDirection,
+    MainAxisAlignment? alignment,
+    MainAxisSize? mainAxisSize,
+    ButtonTextTheme? buttonTextTheme,
+    double? buttonMinWidth,
+    double? buttonHeight,
+    EdgeInsetsGeometry? buttonPadding,
+    bool? buttonAlignedDropdown,
+    ButtonBarLayoutBehavior? layoutBehavior,
+    VerticalDirection? overflowDirection,
   }) {
     return ButtonBarThemeData(
       alignment: alignment ?? this.alignment,
@@ -144,20 +142,20 @@ class ButtonBarThemeData with Diagnosticable {
   /// If both arguments are null, then null is returned.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static ButtonBarThemeData lerp(ButtonBarThemeData a, ButtonBarThemeData b, double t) {
+  static ButtonBarThemeData? lerp(ButtonBarThemeData? a, ButtonBarThemeData? b, double t) {
     assert(t != null);
     if (a == null && b == null)
       return null;
     return ButtonBarThemeData(
-      alignment: t < 0.5 ? a.alignment : b.alignment,
-      mainAxisSize: t < 0.5 ? a.mainAxisSize : b.mainAxisSize,
-      buttonTextTheme: t < 0.5 ? a.buttonTextTheme : b.buttonTextTheme,
+      alignment: t < 0.5 ? a?.alignment : b?.alignment,
+      mainAxisSize: t < 0.5 ? a?.mainAxisSize : b?.mainAxisSize,
+      buttonTextTheme: t < 0.5 ? a?.buttonTextTheme : b?.buttonTextTheme,
       buttonMinWidth: lerpDouble(a?.buttonMinWidth, b?.buttonMinWidth, t),
       buttonHeight: lerpDouble(a?.buttonHeight, b?.buttonHeight, t),
       buttonPadding: EdgeInsetsGeometry.lerp(a?.buttonPadding, b?.buttonPadding, t),
-      buttonAlignedDropdown: t < 0.5 ? a.buttonAlignedDropdown : b.buttonAlignedDropdown,
-      layoutBehavior: t < 0.5 ? a.layoutBehavior : b.layoutBehavior,
-      overflowDirection: t < 0.5 ? a.overflowDirection : b.overflowDirection,
+      buttonAlignedDropdown: t < 0.5 ? a?.buttonAlignedDropdown : b?.buttonAlignedDropdown,
+      layoutBehavior: t < 0.5 ? a?.layoutBehavior : b?.layoutBehavior,
+      overflowDirection: t < 0.5 ? a?.overflowDirection : b?.overflowDirection,
     );
   }
 
@@ -235,9 +233,9 @@ class ButtonBarTheme extends InheritedWidget {
   ///
   /// The [data] must not be null.
   const ButtonBarTheme({
-    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 used for all descendant [ButtonBar] widgets.
@@ -253,8 +251,8 @@ class ButtonBarTheme extends InheritedWidget {
   /// ButtonBarThemeData theme = ButtonBarTheme.of(context);
   /// ```
   static ButtonBarThemeData of(BuildContext context) {
-    final ButtonBarTheme buttonBarTheme = context.dependOnInheritedWidgetOfExactType<ButtonBarTheme>();
-    return buttonBarTheme?.data ?? Theme.of(context).buttonBarTheme;
+    final ButtonBarTheme? buttonBarTheme = context.dependOnInheritedWidgetOfExactType<ButtonBarTheme>();
+    return buttonBarTheme?.data ?? Theme.of(context)!.buttonBarTheme;
   }
 
   @override

packages/flutter/test/material/bottom_navigation_bar_theme_test.dart

@@ -30,7 +30,7 @@ void main() {
     expect(themeData.showUnselectedLabels, null);
     expect(themeData.type, null);
 
-    const BottomNavigationBarTheme theme = BottomNavigationBarTheme(data: BottomNavigationBarThemeData());
+    const BottomNavigationBarTheme theme = BottomNavigationBarTheme(data: BottomNavigationBarThemeData(), child: SizedBox());
     expect(theme.data.backgroundColor, null);
     expect(theme.data.elevation, null);
     expect(theme.data.selectedIconTheme, null);