Normalize Dialog theme (flutter#153982)

This PR is to make preparations to make `DialogTheme` conform to Flutter's conventions for component themes:

* Added a `DialogThemeData` class which defines overrides for the defaults for `Dialog` properties.
* Added 2 `DialogTheme` constructor parameters: `DialogThemeData? data` and `Widget? child`. This is now the preferred way to configure a `DialogTheme`:
```
DialogTheme(
  data: DialogThemeData(color: xxx, elevation: xxx, ...),
  child: Dialog(...)
)
```
  These two properties are made nullable to not break existing apps which has customized `ThemeData.dialogTheme`.

* Changed the type of theme defaults from `DialogTheme` to `DialogThemeData`.

TODO:

* Fix internal failures.
* Change the type of `ThemeData.dialogTheme` from `DialogTheme` to `DialogThemeData`. This may cause breaking changes, a migration guide will be created.

Addresses the "theme normalization" sub project within flutter#91772

Author: QuncCccccc

dev/tools/gen_defaults/lib/dialog_template.dart

@@ -12,7 +12,7 @@ class DialogTemplate extends TokenTemplate {
 
   @override
   String generate() => '''
-class _${blockName}DefaultsM3 extends DialogTheme {
+class _${blockName}DefaultsM3 extends DialogThemeData {
   _${blockName}DefaultsM3(this.context)
     : super(
         alignment: Alignment.center,
@@ -54,7 +54,7 @@ class DialogFullscreenTemplate extends TokenTemplate {
 
   @override
   String generate() => '''
-class _${blockName}DefaultsM3 extends DialogTheme {
+class _${blockName}DefaultsM3 extends DialogThemeData {
   const _${blockName}DefaultsM3(this.context): super(clipBehavior: Clip.none);
 
   final BuildContext context;

packages/flutter/lib/src/material/dialog.dart

@@ -108,7 +108,7 @@ class Dialog extends StatelessWidget {
   /// and a surface tint overlay on the background color if [surfaceTintColor] is
   /// non null.
   ///
-  /// If null then [DialogTheme.elevation] is used, and if that is null then
+  /// If null then [DialogThemeData.elevation] is used, and if that is null then
   /// the elevation will match the Material Design specification for Dialogs.
   ///
   /// See also:
@@ -197,7 +197,7 @@ class Dialog extends StatelessWidget {
   /// See the enum [Clip] for details of all possible options and their common
   /// use cases.
   ///
-  /// If null, then [DialogTheme.clipBehavior] is used. If that is also null,
+  /// If null, then [DialogThemeData.clipBehavior] is used. If that is also null,
   /// defaults to [Clip.none].
   /// {@endtemplate}
   final Clip? clipBehavior;
@@ -214,7 +214,7 @@ class Dialog extends StatelessWidget {
   /// {@template flutter.material.dialog.alignment}
   /// How to align the [Dialog].
   ///
-  /// If null, then [DialogTheme.alignment] is used. If that is also null, the
+  /// If null, then [DialogThemeData.alignment] is used. If that is also null, the
   /// default is [Alignment.center].
   /// {@endtemplate}
   final AlignmentGeometry? alignment;
@@ -230,10 +230,10 @@ class Dialog extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
-    final DialogTheme dialogTheme = DialogTheme.of(context);
+    final DialogThemeData dialogTheme = DialogTheme.of(context).data;
     final EdgeInsets effectivePadding = MediaQuery.viewInsetsOf(context)
       + (insetPadding ?? dialogTheme.insetPadding ?? _defaultInsetPadding);
-    final DialogTheme defaults = theme.useMaterial3
+    final DialogThemeData defaults = theme.useMaterial3
       ? (_fullscreen ? _DialogFullscreenDefaultsM3(context) : _DialogDefaultsM3(context))
       : _DialogDefaultsM2(context);
 
@@ -481,7 +481,7 @@ class AlertDialog extends StatelessWidget {
 
   /// Color for the [Icon] in the [icon] of this [AlertDialog].
   ///
-  /// If null, [DialogTheme.iconColor] is used. If that is null, defaults to
+  /// If null, [DialogThemeData.iconColor] is used. If that is null, defaults to
   /// color scheme's [ColorScheme.secondary] if [ThemeData.useMaterial3] is
   /// true, black otherwise.
   final Color? iconColor;
@@ -519,7 +519,7 @@ class AlertDialog extends StatelessWidget {
 
   /// Style for the text in the [title] of this [AlertDialog].
   ///
-  /// If null, [DialogTheme.titleTextStyle] is used. If that's null, defaults to
+  /// If null, [DialogThemeData.titleTextStyle] is used. If that's null, defaults to
   /// [TextTheme.headlineSmall] of [ThemeData.textTheme] if
   /// [ThemeData.useMaterial3] is true, [TextTheme.titleLarge] otherwise.
   final TextStyle? titleTextStyle;
@@ -554,7 +554,7 @@ class AlertDialog extends StatelessWidget {
 
   /// Style for the text in the [content] of this [AlertDialog].
   ///
-  /// If null, [DialogTheme.contentTextStyle] is used. If that's null, defaults
+  /// If null, [DialogThemeData.contentTextStyle] is used. If that's null, defaults
   /// to [TextTheme.bodyMedium] of [ThemeData.textTheme] if
   /// [ThemeData.useMaterial3] is true, [TextTheme.titleMedium] otherwise.
   final TextStyle? contentTextStyle;
@@ -720,8 +720,8 @@ class AlertDialog extends StatelessWidget {
     assert(debugCheckHasMaterialLocalizations(context));
     final ThemeData theme = Theme.of(context);
 
-    final DialogTheme dialogTheme = DialogTheme.of(context);
-    final DialogTheme defaults = theme.useMaterial3 ? _DialogDefaultsM3(context) : _DialogDefaultsM2(context);
+    final DialogThemeData dialogTheme = DialogTheme.of(context).data;
+    final DialogThemeData defaults = theme.useMaterial3 ? _DialogDefaultsM3(context) : _DialogDefaultsM2(context);
 
     String? label = semanticLabel;
     switch (theme.platform) {
@@ -1155,7 +1155,7 @@ class SimpleDialog extends StatelessWidget {
 
   /// Style for the text in the [title] of this [SimpleDialog].
   ///
-  /// If null, [DialogTheme.titleTextStyle] is used. If that's null, defaults to
+  /// If null, [DialogThemeData.titleTextStyle] is used. If that's null, defaults to
   /// [TextTheme.titleLarge] of [ThemeData.textTheme].
   final TextStyle? titleTextStyle;
 
@@ -1235,7 +1235,7 @@ class SimpleDialog extends StatelessWidget {
 
     // The paddingScaleFactor is used to adjust the padding of Dialog
     // children.
-    final TextStyle defaultTextStyle = titleTextStyle ?? DialogTheme.of(context).titleTextStyle ?? theme.textTheme.titleLarge!;
+    final TextStyle defaultTextStyle = titleTextStyle ?? DialogTheme.of(context).data.titleTextStyle ?? theme.textTheme.titleLarge!;
     final double fontSize = defaultTextStyle.fontSize ?? kDefaultFontSize;
     final double fontSizeToScale = fontSize == 0.0 ? kDefaultFontSize : fontSize;
     final double effectiveTextScale = MediaQuery.textScalerOf(context).scale(fontSizeToScale) / fontSizeToScale;
@@ -1342,7 +1342,7 @@ Widget _buildMaterialDialogTransitions(BuildContext context, Animation<double> a
 ///
 /// The `barrierColor` argument is used to specify the color of the modal
 /// barrier that darkens everything below the dialog. If `null` the `barrierColor`
-/// field from `DialogTheme` is used. If that is `null` the default color
+/// field from `DialogThemeData` is used. If that is `null` the default color
 /// `Colors.black54` is used.
 ///
 /// The `useSafeArea` argument is used to indicate if the dialog should only
@@ -1443,7 +1443,10 @@ Future<T?> showDialog<T>({
   return Navigator.of(context, rootNavigator: useRootNavigator).push<T>(DialogRoute<T>(
     context: context,
     builder: builder,
-    barrierColor: barrierColor ?? Theme.of(context).dialogTheme.barrierColor ?? Colors.black54,
+    barrierColor: barrierColor
+      ?? DialogTheme.of(context).barrierColor
+      ?? Theme.of(context).dialogTheme.data.barrierColor
+      ?? Colors.black54,
     barrierDismissible: barrierDismissible,
     barrierLabel: barrierLabel,
     useSafeArea: useSafeArea,
@@ -1633,7 +1636,7 @@ double _scalePadding(double textScaleFactor) {
 }
 
 // Hand coded defaults based on Material Design 2.
-class _DialogDefaultsM2 extends DialogTheme {
+class _DialogDefaultsM2 extends DialogThemeData {
   _DialogDefaultsM2(this.context)
     : _textTheme = Theme.of(context).textTheme,
       _iconTheme = Theme.of(context).iconTheme,
@@ -1674,7 +1677,7 @@ class _DialogDefaultsM2 extends DialogTheme {
 // Design token database by the script:
 //   dev/tools/gen_defaults/bin/gen_defaults.dart.
 
-class _DialogDefaultsM3 extends DialogTheme {
+class _DialogDefaultsM3 extends DialogThemeData {
   _DialogDefaultsM3(this.context)
     : super(
         alignment: Alignment.center,
@@ -1718,7 +1721,7 @@ class _DialogDefaultsM3 extends DialogTheme {
 // Design token database by the script:
 //   dev/tools/gen_defaults/bin/gen_defaults.dart.
 
-class _DialogFullscreenDefaultsM3 extends DialogTheme {
+class _DialogFullscreenDefaultsM3 extends DialogThemeData {
   const _DialogFullscreenDefaultsM3(this.context): super(clipBehavior: Clip.none);
 
   final BuildContext context;