migrate some cupertino files to nullsafety (#65880)

packages/flutter/lib/src/cupertino/colors.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, Brightness;
 
 import '../../foundation.dart';
@@ -683,15 +681,15 @@ class CupertinoDynamicColor extends Color with Diagnosticable {
   ///
   /// All the colors must not be null.
   const CupertinoDynamicColor({
-    String debugLabel,
-    @required Color color,
-    @required Color darkColor,
-    @required Color highContrastColor,
-    @required Color darkHighContrastColor,
-    @required Color elevatedColor,
-    @required Color darkElevatedColor,
-    @required Color highContrastElevatedColor,
-    @required Color darkHighContrastElevatedColor,
+    String? debugLabel,
+    required Color color,
+    required Color darkColor,
+    required Color highContrastColor,
+    required Color darkHighContrastColor,
+    required Color elevatedColor,
+    required Color darkElevatedColor,
+    required Color highContrastElevatedColor,
+    required Color darkHighContrastElevatedColor,
   }) : this._(
          color,
          color,
@@ -713,11 +711,11 @@ class CupertinoDynamicColor extends Color with Diagnosticable {
   ///
   /// All the colors must not be null.
   const CupertinoDynamicColor.withBrightnessAndContrast({
-    String debugLabel,
-    @required Color color,
-    @required Color darkColor,
-    @required Color highContrastColor,
-    @required Color darkHighContrastColor,
+    String? debugLabel,
+    required Color color,
+    required Color darkColor,
+    required Color highContrastColor,
+    required Color darkHighContrastColor,
   }) : this(
     debugLabel: debugLabel,
     color: color,
@@ -736,9 +734,9 @@ class CupertinoDynamicColor extends Color with Diagnosticable {
   ///
   /// All the colors must not be null.
   const CupertinoDynamicColor.withBrightness({
-    String debugLabel,
-    @required Color color,
-    @required Color darkColor,
+    String? debugLabel,
+    required Color color,
+    required Color darkColor,
   }) : this(
     debugLabel: debugLabel,
     color: color,
@@ -786,9 +784,9 @@ class CupertinoDynamicColor extends Color with Diagnosticable {
   @override
   int get value => _effectiveColor.value;
 
-  final String _debugLabel;
+  final String? _debugLabel;
 
-  final Element _debugResolveContext;
+  final Element? _debugResolveContext;
 
   /// The color to use when the [BuildContext] implies a combination of light mode,
   /// normal contrast, and base interface elevation.
@@ -887,7 +885,7 @@ class CupertinoDynamicColor extends Color with Diagnosticable {
   /// value will be used ([Brightness.light] platform brightness, normal contrast,
   /// [CupertinoUserInterfaceLevelData.base] elevation level), unless [nullOk] is
   /// set to false, in which case an exception will be thrown.
-  static Color resolve(Color resolvable, BuildContext context, { bool nullOk = true }) {
+  static Color? resolve(Color? resolvable, BuildContext context, { bool nullOk = true }) {
     if (resolvable == null)
       return null;
     assert(context != null);
@@ -981,7 +979,7 @@ class CupertinoDynamicColor extends Color with Diagnosticable {
         }
     }
 
-    Element _debugContext;
+    Element? _debugContext;
     assert(() {
       _debugContext = context as Element;
       return true;
@@ -1058,7 +1056,7 @@ class CupertinoDynamicColor extends Color with Diagnosticable {
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     if (_debugLabel != null)
-      properties.add(MessageProperty('debugLabel', _debugLabel));
+      properties.add(MessageProperty('debugLabel', _debugLabel!));
     properties.add(createCupertinoColorProperty('color', color));
     if (_isPlatformBrightnessDependent)
       properties.add(createCupertinoColorProperty('darkColor', darkColor));
@@ -1085,11 +1083,11 @@ class CupertinoDynamicColor extends Color with Diagnosticable {
 /// The [showName], [style], and [level] arguments must not be null.
 DiagnosticsProperty<Color> createCupertinoColorProperty(
   String name,
-  Color value, {
-    bool showName = true,
-    Object defaultValue = kNoDefaultValue,
-    DiagnosticsTreeStyle style = DiagnosticsTreeStyle.singleLine,
-    DiagnosticLevel level = DiagnosticLevel.info,
+  Color? value, {
+  bool showName = true,
+  Object? defaultValue = kNoDefaultValue,
+  DiagnosticsTreeStyle style = DiagnosticsTreeStyle.singleLine,
+  DiagnosticLevel level = DiagnosticLevel.info,
 }) {
   if (value is CupertinoDynamicColor) {
     return DiagnosticsProperty<CupertinoDynamicColor>(

packages/flutter/lib/src/cupertino/icon_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 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 import 'colors.dart';
@@ -16,22 +14,22 @@ class CupertinoIconThemeData extends IconThemeData with Diagnosticable {
   /// The opacity applies to both explicit and default icon colors. The value
   /// is clamped between 0.0 and 1.0.
   const CupertinoIconThemeData({
-    Color color,
-    double opacity,
-    double size
+    Color? color,
+    double? opacity,
+    double? size,
   }) : super(color: color, opacity: opacity, size: size);
 
   /// Called by [IconTheme.of] to resolve [color] against the given [BuildContext].
   @override
   IconThemeData resolve(BuildContext context) {
-    final Color resolvedColor = CupertinoDynamicColor.resolve(color, context);
+    final Color? resolvedColor = CupertinoDynamicColor.resolve(color, context);
     return resolvedColor == color ? this : copyWith(color: resolvedColor);
   }
 
   /// Creates a copy of this icon theme but with the given fields replaced with
   /// the new values.
   @override
-  CupertinoIconThemeData copyWith({ Color color, double opacity, double size }) {
+  CupertinoIconThemeData copyWith({ Color? color, double? opacity, double? size }) {
     return CupertinoIconThemeData(
       color: color ?? this.color,
       opacity: opacity ?? this.opacity,

packages/flutter/lib/src/cupertino/interface_level.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 '../widgets/framework.dart';
@@ -41,9 +39,9 @@ class CupertinoUserInterfaceLevel extends InheritedWidget {
   /// Creates a [CupertinoUserInterfaceLevel] to change descendant Cupertino widget's
   /// visual level.
   const CupertinoUserInterfaceLevel({
-    Key key,
-    @required CupertinoUserInterfaceLevelData data,
-    Widget child,
+    Key? key,
+    required CupertinoUserInterfaceLevelData data,
+    required Widget child,
   }) : assert(data != null),
       _data = data,
       super(key: key, child: child);
@@ -59,10 +57,10 @@ class CupertinoUserInterfaceLevel extends InheritedWidget {
   /// You can use this function to query the user interface elevation level within
   /// the given [BuildContext]. When that information changes, your widget will
   /// be scheduled to be rebuilt, keeping your widget up-to-date.
-  static CupertinoUserInterfaceLevelData of(BuildContext context, { bool nullOk = false }) {
+  static CupertinoUserInterfaceLevelData? of(BuildContext context, { bool nullOk = false }) {
     assert(context != null);
     assert(nullOk != null);
-    final CupertinoUserInterfaceLevel query = context.dependOnInheritedWidgetOfExactType<CupertinoUserInterfaceLevel>();
+    final CupertinoUserInterfaceLevel? query = context.dependOnInheritedWidgetOfExactType<CupertinoUserInterfaceLevel>();
     if (query != null)
       return query._data;
     if (nullOk)

packages/flutter/lib/src/cupertino/text_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/services.dart' show Brightness;
 import 'package:flutter/widgets.dart';
@@ -98,12 +96,12 @@ const TextStyle _kDefaultDateTimePickerTextStyle = TextStyle(
   color: CupertinoColors.label,
 );
 
-TextStyle _resolveTextStyle(TextStyle style, BuildContext context, bool nullOk) {
+TextStyle? _resolveTextStyle(TextStyle? style, BuildContext context, bool nullOk) {
   // This does not resolve the shadow color, foreground, background, etc.
   return style?.copyWith(
-    color: CupertinoDynamicColor.resolve(style?.color, context, nullOk: nullOk),
-    backgroundColor: CupertinoDynamicColor.resolve(style?.backgroundColor, context, nullOk: nullOk),
-    decorationColor: CupertinoDynamicColor.resolve(style?.decorationColor, context, nullOk: nullOk),
+    color: CupertinoDynamicColor.resolve(style.color, context, nullOk: nullOk),
+    backgroundColor: CupertinoDynamicColor.resolve(style.backgroundColor, context, nullOk: nullOk),
+    decorationColor: CupertinoDynamicColor.resolve(style.decorationColor, context, nullOk: nullOk),
   );
 }
 
@@ -126,15 +124,15 @@ class CupertinoTextThemeData with Diagnosticable {
       'This argument no longer does anything. You can remove it. '
       'This feature was deprecated after v1.10.14.'
     )
-    Brightness brightness,
-    TextStyle textStyle,
-    TextStyle actionTextStyle,
-    TextStyle tabLabelTextStyle,
-    TextStyle navTitleTextStyle,
-    TextStyle navLargeTitleTextStyle,
-    TextStyle navActionTextStyle,
-    TextStyle pickerTextStyle,
-    TextStyle dateTimePickerTextStyle,
+    Brightness? brightness,
+    TextStyle? textStyle,
+    TextStyle? actionTextStyle,
+    TextStyle? tabLabelTextStyle,
+    TextStyle? navTitleTextStyle,
+    TextStyle? navLargeTitleTextStyle,
+    TextStyle? navActionTextStyle,
+    TextStyle? pickerTextStyle,
+    TextStyle? dateTimePickerTextStyle,
   }) : this._raw(
          const _TextThemeDefaultsBuilder(CupertinoColors.label, CupertinoColors.inactiveGray),
          primaryColor,
@@ -162,41 +160,41 @@ class CupertinoTextThemeData with Diagnosticable {
   ) : assert((_navActionTextStyle != null && _actionTextStyle != null) || _primaryColor != null);
 
   final _TextThemeDefaultsBuilder _defaults;
-  final Color _primaryColor;
+  final Color? _primaryColor;
 
-  final TextStyle _textStyle;
+  final TextStyle? _textStyle;
   /// The [TextStyle] of general text content for Cupertino widgets.
   TextStyle get textStyle => _textStyle ?? _defaults.textStyle;
 
-  final TextStyle _actionTextStyle;
+  final TextStyle? _actionTextStyle;
   /// The [TextStyle] of interactive text content such as text in a button without background.
   TextStyle get actionTextStyle {
     return _actionTextStyle ?? _defaults.actionTextStyle(primaryColor: _primaryColor);
   }
 
-  final TextStyle _tabLabelTextStyle;
+  final TextStyle? _tabLabelTextStyle;
   /// The [TextStyle] of unselected tabs.
   TextStyle get tabLabelTextStyle => _tabLabelTextStyle ?? _defaults.tabLabelTextStyle;
 
-  final TextStyle _navTitleTextStyle;
+  final TextStyle? _navTitleTextStyle;
   /// The [TextStyle] of titles in standard navigation bars.
   TextStyle get navTitleTextStyle => _navTitleTextStyle ?? _defaults.navTitleTextStyle;
 
-  final TextStyle _navLargeTitleTextStyle;
+  final TextStyle? _navLargeTitleTextStyle;
   /// The [TextStyle] of large titles in sliver navigation bars.
   TextStyle get navLargeTitleTextStyle => _navLargeTitleTextStyle ?? _defaults.navLargeTitleTextStyle;
 
-  final TextStyle _navActionTextStyle;
+  final TextStyle? _navActionTextStyle;
   /// The [TextStyle] of interactive text content in navigation bars.
   TextStyle get navActionTextStyle {
     return _navActionTextStyle ?? _defaults.navActionTextStyle(primaryColor: _primaryColor);
   }
 
-  final TextStyle _pickerTextStyle;
+  final TextStyle? _pickerTextStyle;
   /// The [TextStyle] of pickers.
   TextStyle get pickerTextStyle => _pickerTextStyle ?? _defaults.pickerTextStyle;
 
-  final TextStyle _dateTimePickerTextStyle;
+  final TextStyle? _dateTimePickerTextStyle;
   /// The [TextStyle] of date time pickers.
   TextStyle get dateTimePickerTextStyle => _dateTimePickerTextStyle ?? _defaults.dateTimePickerTextStyle;
 
@@ -209,7 +207,7 @@ class CupertinoTextThemeData with Diagnosticable {
   /// be used as-is.
   CupertinoTextThemeData resolveFrom(BuildContext context, { bool nullOk = false }) {
     return CupertinoTextThemeData._raw(
-      _defaults?.resolveFrom(context, nullOk),
+      _defaults.resolveFrom(context, nullOk),
       CupertinoDynamicColor.resolve(_primaryColor, context, nullOk: nullOk),
       _resolveTextStyle(_textStyle, context, nullOk),
       _resolveTextStyle(_actionTextStyle, context, nullOk),
@@ -225,20 +223,20 @@ class CupertinoTextThemeData with Diagnosticable {
   /// Returns a copy of the current [CupertinoTextThemeData] instance with
   /// specified overrides.
   CupertinoTextThemeData copyWith({
-    Color primaryColor,
+    Color? primaryColor,
     @Deprecated(
       'This argument no longer does anything. You can remove it. '
       'This feature was deprecated after v1.10.14.'
     )
-    Brightness brightness,
-    TextStyle textStyle,
-    TextStyle actionTextStyle,
-    TextStyle tabLabelTextStyle,
-    TextStyle navTitleTextStyle,
-    TextStyle navLargeTitleTextStyle,
-    TextStyle navActionTextStyle,
-    TextStyle pickerTextStyle,
-    TextStyle dateTimePickerTextStyle,
+    Brightness? brightness,
+    TextStyle? textStyle,
+    TextStyle? actionTextStyle,
+    TextStyle? tabLabelTextStyle,
+    TextStyle? navTitleTextStyle,
+    TextStyle? navLargeTitleTextStyle,
+    TextStyle? navActionTextStyle,
+    TextStyle? pickerTextStyle,
+    TextStyle? dateTimePickerTextStyle,
   }) {
     return CupertinoTextThemeData._raw(
       _defaults,
@@ -282,9 +280,9 @@ class _TextThemeDefaultsBuilder {
   final Color inactiveGrayColor;
 
   static TextStyle _applyLabelColor(TextStyle original, Color color) {
-    return original?.color == color
+    return original.color == color
       ?  original
-      :  original?.copyWith(color: color);
+      :  original.copyWith(color: color);
   }
 
   TextStyle get textStyle => _applyLabelColor(_kDefaultTextStyle, labelColor);
@@ -294,12 +292,12 @@ class _TextThemeDefaultsBuilder {
   TextStyle get pickerTextStyle => _applyLabelColor(_kDefaultPickerTextStyle, labelColor);
   TextStyle get dateTimePickerTextStyle => _applyLabelColor(_kDefaultDateTimePickerTextStyle, labelColor);
 
-  TextStyle actionTextStyle({ Color primaryColor }) => _kDefaultActionTextStyle.copyWith(color: primaryColor);
-  TextStyle navActionTextStyle({ Color primaryColor }) => actionTextStyle(primaryColor: primaryColor);
+  TextStyle actionTextStyle({ Color? primaryColor }) => _kDefaultActionTextStyle.copyWith(color: primaryColor);
+  TextStyle navActionTextStyle({ Color? primaryColor }) => actionTextStyle(primaryColor: primaryColor);
 
   _TextThemeDefaultsBuilder resolveFrom(BuildContext context, bool nullOk) {
-    final Color resolvedLabelColor = CupertinoDynamicColor.resolve(labelColor, context, nullOk: nullOk);
-    final Color resolvedInactiveGray = CupertinoDynamicColor.resolve(inactiveGrayColor, context, nullOk: nullOk);
+    final Color resolvedLabelColor = CupertinoDynamicColor.resolve(labelColor, context, nullOk: nullOk)!;
+    final Color resolvedInactiveGray = CupertinoDynamicColor.resolve(inactiveGrayColor, context, nullOk: nullOk)!;
     return resolvedLabelColor == labelColor && resolvedInactiveGray == CupertinoColors.inactiveGray
       ? this
       : _TextThemeDefaultsBuilder(resolvedLabelColor, resolvedInactiveGray);

packages/flutter/lib/src/cupertino/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/services.dart';
 import 'package:flutter/widgets.dart';
@@ -52,9 +50,9 @@ class CupertinoTheme extends StatelessWidget {
   ///
   /// The [data] and [child] parameters must not be null.
   const CupertinoTheme({
-    Key key,
-    @required this.data,
-    @required this.child,
+    Key? key,
+    required this.data,
+    required this.child,
   }) : assert(child != null),
        assert(data != null),
        super(key: key);
@@ -69,8 +67,8 @@ class CupertinoTheme extends StatelessWidget {
   /// Resolves all the colors defined in that [CupertinoThemeData] against the
   /// given [BuildContext] on a best-effort basis.
   static CupertinoThemeData of(BuildContext context) {
-    final _InheritedCupertinoTheme inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedCupertinoTheme>();
-    return (inheritedTheme?.theme?.data ?? const CupertinoThemeData()).resolveFrom(context, nullOk: true);
+    final _InheritedCupertinoTheme? inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedCupertinoTheme>();
+    return (inheritedTheme?.theme.data ?? const CupertinoThemeData()).resolveFrom(context, nullOk: true);
   }
 
   /// Retrieves the [Brightness] to use for descendant Cupertino widgets, based
@@ -86,9 +84,9 @@ class CupertinoTheme extends StatelessWidget {
   ///
   /// * [CupertinoThemeData.brightness], the property takes precedence over
   ///   [MediaQueryData.platformBrightness] for descendant Cupertino widgets.
-  static Brightness brightnessOf(BuildContext context, { bool nullOk = false }) {
-    final _InheritedCupertinoTheme inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedCupertinoTheme>();
-    return inheritedTheme?.theme?.data?.brightness ?? MediaQuery.of(context, nullOk: nullOk)?.platformBrightness;
+  static Brightness? brightnessOf(BuildContext context, { bool nullOk = false }) {
+    final _InheritedCupertinoTheme? inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedCupertinoTheme>();
+    return inheritedTheme?.theme.data.brightness ?? MediaQuery.of(context, nullOk: nullOk)?.platformBrightness;
   }
 
   /// The widget below this widget in the tree.
@@ -116,9 +114,9 @@ class CupertinoTheme extends StatelessWidget {
 
 class _InheritedCupertinoTheme extends InheritedWidget {
   const _InheritedCupertinoTheme({
-    Key key,
-    @required this.theme,
-    @required Widget child,
+    Key? key,
+    required this.theme,
+    required Widget child,
   }) : assert(theme != null),
        super(key: key, child: child);
 
@@ -150,12 +148,12 @@ class CupertinoThemeData with Diagnosticable {
   ///
   /// Unspecified parameters default to a reasonable iOS default style.
   const CupertinoThemeData({
-    Brightness brightness,
-    Color primaryColor,
-    Color primaryContrastingColor,
-    CupertinoTextThemeData textTheme,
-    Color barBackgroundColor,
-    Color scaffoldBackgroundColor,
+    Brightness? brightness,
+    Color? primaryColor,
+    Color? primaryContrastingColor,
+    CupertinoTextThemeData? textTheme,
+    Color? barBackgroundColor,
+    Color? scaffoldBackgroundColor,
   }) : this.raw(
         brightness,
         primaryColor,
@@ -171,12 +169,12 @@ class CupertinoThemeData with Diagnosticable {
   /// Used by subclasses to get the superclass's defaulting behaviors.
   @protected
   const CupertinoThemeData.raw(
-    Brightness brightness,
-    Color primaryColor,
-    Color primaryContrastingColor,
-    CupertinoTextThemeData textTheme,
-    Color barBackgroundColor,
-    Color scaffoldBackgroundColor,
+    Brightness? brightness,
+    Color? primaryColor,
+    Color? primaryContrastingColor,
+    CupertinoTextThemeData? textTheme,
+    Color? barBackgroundColor,
+    Color? scaffoldBackgroundColor,
   ) : this._rawWithDefaults(
     brightness,
     primaryColor,
@@ -197,7 +195,7 @@ class CupertinoThemeData with Diagnosticable {
     this._defaults,
   );
 
-  final _CupertinoThemeDefaults _defaults;
+  final _CupertinoThemeDefaults? _defaults;
 
   /// The brightness override for Cupertino descendants.
   ///
@@ -215,7 +213,7 @@ class CupertinoThemeData with Diagnosticable {
   ///
   ///  * [CupertinoTheme.brightnessOf], a method used to retrieve the overall
   ///    [Brightness] from a [BuildContext], for Cupertino widgets.
-  final Brightness brightness;
+  final Brightness? brightness;
 
   /// A color used on interactive elements of the theme.
   ///
@@ -232,8 +230,8 @@ class CupertinoThemeData with Diagnosticable {
   ///
   ///  * [MaterialBasedCupertinoThemeData], a [CupertinoThemeData] that defers
   ///    [primaryColor] to its Material [Theme] parent if it's unspecified.
-  Color get primaryColor => _primaryColor ?? _defaults.primaryColor;
-  final Color _primaryColor;
+  Color? get primaryColor => _primaryColor ?? _defaults!.primaryColor;
+  final Color? _primaryColor;
 
   /// A color that must be easy to see when rendered on a [primaryColor] background.
   ///
@@ -247,29 +245,29 @@ class CupertinoThemeData with Diagnosticable {
   ///
   ///  * [MaterialBasedCupertinoThemeData], a [CupertinoThemeData] that defers
   ///    [primaryContrastingColor] to its Material [Theme] parent if it's unspecified.
-  Color get primaryContrastingColor => _primaryContrastingColor ?? _defaults.primaryContrastingColor;
-  final Color _primaryContrastingColor;
+  Color? get primaryContrastingColor => _primaryContrastingColor ?? _defaults!.primaryContrastingColor;
+  final Color? _primaryContrastingColor;
 
   /// Text styles used by Cupertino widgets.
   ///
   /// Derived from [primaryColor] if unspecified.
-  CupertinoTextThemeData get textTheme {
-    return _textTheme ?? _defaults.textThemeDefaults.createDefaults(primaryColor: primaryColor);
+  CupertinoTextThemeData? get textTheme {
+    return _textTheme ?? _defaults!.textThemeDefaults.createDefaults(primaryColor: primaryColor!);
   }
-  final CupertinoTextThemeData _textTheme;
+  final CupertinoTextThemeData? _textTheme;
 
   /// Background color of the top nav bar and bottom tab bar.
   ///
   /// Defaults to a light gray in light mode, or a dark translucent gray color in
   /// dark mode.
-  Color get barBackgroundColor => _barBackgroundColor ?? _defaults.barBackgroundColor;
-  final Color _barBackgroundColor;
+  Color? get barBackgroundColor => _barBackgroundColor ?? _defaults!.barBackgroundColor;
+  final Color? _barBackgroundColor;
 
   /// Background color of the scaffold.
   ///
   /// Defaults to [CupertinoColors.systemBackground].
-  Color get scaffoldBackgroundColor => _scaffoldBackgroundColor ?? _defaults.scaffoldBackgroundColor;
-  final Color _scaffoldBackgroundColor;
+  Color? get scaffoldBackgroundColor => _scaffoldBackgroundColor ?? _defaults!.scaffoldBackgroundColor;
+  final Color? _scaffoldBackgroundColor;
 
   /// Returns an instance of the [CupertinoThemeData] whose property getters
   /// only return the construction time specifications with no derived values.
@@ -294,7 +292,7 @@ class CupertinoThemeData with Diagnosticable {
   /// [CupertinoThemeData].
   @protected
   CupertinoThemeData resolveFrom(BuildContext context, { bool nullOk = false }) {
-    Color convertColor(Color color) => CupertinoDynamicColor.resolve(color, context, nullOk: nullOk);
+    Color? convertColor(Color? color) => CupertinoDynamicColor.resolve(color, context, nullOk: nullOk);
 
     return CupertinoThemeData._rawWithDefaults(
       brightness,
@@ -303,7 +301,7 @@ class CupertinoThemeData with Diagnosticable {
       _textTheme?.resolveFrom(context, nullOk: nullOk),
       convertColor(_barBackgroundColor),
       convertColor(_scaffoldBackgroundColor),
-      _defaults.resolveFrom(context, _textTheme == null, nullOk: nullOk),
+      _defaults!.resolveFrom(context, _textTheme == null, nullOk: nullOk),
     );
   }
 
@@ -315,12 +313,12 @@ class CupertinoThemeData with Diagnosticable {
   /// copying with a different [primaryColor] will also change the copy's implied
   /// [textTheme].
   CupertinoThemeData 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 CupertinoThemeData._rawWithDefaults(
       brightness ?? this.brightness,
@@ -342,13 +340,13 @@ class CupertinoThemeData with Diagnosticable {
     properties.add(createCupertinoColorProperty('primaryContrastingColor', primaryContrastingColor, defaultValue: defaultData.primaryContrastingColor));
     properties.add(createCupertinoColorProperty('barBackgroundColor', barBackgroundColor, defaultValue: defaultData.barBackgroundColor));
     properties.add(createCupertinoColorProperty('scaffoldBackgroundColor', scaffoldBackgroundColor, defaultValue: defaultData.scaffoldBackgroundColor));
-    textTheme.debugFillProperties(properties);
+    textTheme!.debugFillProperties(properties);
   }
 }
 
 class _NoDefaultCupertinoThemeData extends CupertinoThemeData {
   const _NoDefaultCupertinoThemeData(
-    Brightness brightness,
+    Brightness? brightness,
     this.primaryColor,
     this.primaryContrastingColor,
     this.textTheme,
@@ -365,19 +363,19 @@ class _NoDefaultCupertinoThemeData extends CupertinoThemeData {
       );
 
   @override
-  final Color primaryColor;
+  final Color? primaryColor;
   @override
-  final Color primaryContrastingColor;
+  final Color? primaryContrastingColor;
   @override
-  final CupertinoTextThemeData textTheme;
+  final CupertinoTextThemeData? textTheme;
   @override
-  final Color barBackgroundColor;
+  final Color? barBackgroundColor;
   @override
-  final Color scaffoldBackgroundColor;
+  final Color? scaffoldBackgroundColor;
 
   @override
   _NoDefaultCupertinoThemeData resolveFrom(BuildContext context, { bool nullOk = false }) {
-    Color convertColor(Color color) => CupertinoDynamicColor.resolve(color, context, nullOk: nullOk);
+    Color? convertColor(Color? color) => CupertinoDynamicColor.resolve(color, context, nullOk: nullOk);
 
     return _NoDefaultCupertinoThemeData(
       brightness,
@@ -391,12 +389,12 @@ class _NoDefaultCupertinoThemeData extends CupertinoThemeData {
 
   @override
   CupertinoThemeData 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 _NoDefaultCupertinoThemeData(
       brightness ?? this.brightness,
@@ -420,16 +418,16 @@ class _CupertinoThemeDefaults {
     this.textThemeDefaults,
   );
 
-  final Brightness brightness;
+  final Brightness? brightness;
   final Color primaryColor;
   final Color primaryContrastingColor;
   final Color barBackgroundColor;
   final Color scaffoldBackgroundColor;
   final _CupertinoTextThemeDefaults textThemeDefaults;
 
-  _CupertinoThemeDefaults resolveFrom(BuildContext context, bool resolveTextTheme, { @required bool nullOk }) {
+  _CupertinoThemeDefaults resolveFrom(BuildContext context, bool resolveTextTheme, { required bool nullOk }) {
     assert(nullOk != null);
-    Color convertColor(Color color) => CupertinoDynamicColor.resolve(color, context, nullOk: nullOk);
+    Color convertColor(Color color) => CupertinoDynamicColor.resolve(color, context, nullOk: nullOk)!;
 
     return _CupertinoThemeDefaults(
       brightness,
@@ -437,7 +435,7 @@ class _CupertinoThemeDefaults {
       convertColor(primaryContrastingColor),
       convertColor(barBackgroundColor),
       convertColor(scaffoldBackgroundColor),
-      resolveTextTheme ? textThemeDefaults?.resolveFrom(context, nullOk: nullOk) : textThemeDefaults,
+      resolveTextTheme ? textThemeDefaults.resolveFrom(context, nullOk: nullOk) : textThemeDefaults,
     );
   }
 }
@@ -452,14 +450,14 @@ class _CupertinoTextThemeDefaults {
   final Color labelColor;
   final Color inactiveGray;
 
-  _CupertinoTextThemeDefaults resolveFrom(BuildContext context, { @required bool nullOk }) {
+  _CupertinoTextThemeDefaults resolveFrom(BuildContext context, { required bool nullOk }) {
     return _CupertinoTextThemeDefaults(
-      CupertinoDynamicColor.resolve(labelColor, context, nullOk: nullOk),
-      CupertinoDynamicColor.resolve(inactiveGray, context, nullOk: nullOk),
+      CupertinoDynamicColor.resolve(labelColor, context, nullOk: nullOk)!,
+      CupertinoDynamicColor.resolve(inactiveGray, context, nullOk: nullOk)!,
     );
   }
 
-  CupertinoTextThemeData createDefaults({ @required Color primaryColor }) {
+  CupertinoTextThemeData createDefaults({ required Color primaryColor }) {
     assert(primaryColor != null);
     return _DefaultCupertinoTextThemeData(
       primaryColor: primaryColor,
@@ -474,9 +472,9 @@ class _CupertinoTextThemeDefaults {
 // text styles changes.
 class _DefaultCupertinoTextThemeData extends CupertinoTextThemeData {
   const _DefaultCupertinoTextThemeData({
-    @required this.labelColor,
-    @required this.inactiveGray,
-    @required Color primaryColor,
+    required this.labelColor,
+    required this.inactiveGray,
+    required Color primaryColor,
   }) : assert(labelColor != null),
        assert(inactiveGray != null),
        assert(primaryColor != null),

packages/flutter/test/cupertino/colors_test.dart

@@ -167,7 +167,7 @@ void main() {
   });
 
   test('can resolve null color', () {
-    expect(CupertinoDynamicColor.resolve(null, null), isNull);
+    expect(CupertinoDynamicColor.resolve(null, _NullElement.instance), isNull);
   });
 
   test('withVibrancy constructor creates colors that may depend on vibrancy', () {
@@ -589,3 +589,20 @@ void main() {
     });
   });
 }
+
+class _NullElement extends Element {
+  _NullElement() : super(_NullWidget());
+
+  static _NullElement instance = _NullElement();
+
+  @override
+  bool get debugDoingBuild => throw UnimplementedError();
+
+  @override
+  void performRebuild() { }
+}
+
+class _NullWidget extends Widget {
+  @override
+  Element createElement() => throw UnimplementedError();
+}
\ No newline at end of file