Migration guide for the material theme system updates (#11707)

This PR is to add a breaking change page for the update in the Material
Theme system. Related to flutter/flutter#157292

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- [ ] This PR is marked as draft with an explanation if not meant to
land until a future stable release.
- [x] This PR doesn’t contain automatically generated corrections
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---------

Co-authored-by: Shams Zakhour (ignore Sfshaza) <[email protected]>

Author: QuncCccccc

src/content/release/breaking-changes/index.md

@@ -42,6 +42,7 @@ They're sorted by release and listed in alphabetical order:
 * [Deprecate `ThemeData.indicatorColor` in favor of `TabBarThemeData.indicatorColor`][]
 * [`ImageFilter.blur` default tile mode automatic selection][]
 * [Localized messages are generated into source, not a synthetic package][]
+* [Material Theme System Updates][]
 * [Stop generating `AssetManifest.json`][]
 * [Updated Material 3 `Slider`][]
 * [Updated Material 3 progress indicators][]
@@ -54,6 +55,7 @@ They're sorted by release and listed in alphabetical order:
 [Deprecate `ThemeData.indicatorColor` in favor of `TabBarThemeData.indicatorColor`]: /release/breaking-changes/deprecate-themedata-indicatorcolor
 [`ImageFilter.blur` default tile mode automatic selection]: /release/breaking-changes/image-filter-blur-tilemode
 [Localized messages are generated into source, not a synthetic package]: /release/breaking-changes/flutter-generate-i10n-source
+[Material Theme System Updates]: /release/breaking-changes/material-theme-system-updates
 [Stop generating `AssetManifest.json`]: /release/breaking-changes/asset-manifest-dot-json
 [Updated Material 3 `Slider`]: /release/breaking-changes/updated-material-3-slider
 [Updated Material 3 progress indicators]: /release/breaking-changes/updated-material-3-progress-indicators

src/content/release/breaking-changes/material-theme-system-updates.md

@@ -0,0 +1,76 @@
+---
+title: Material Theme System Updates
+description: >-
+  `CardTheme`, `DialogTheme`, and `TabBarTheme` have been normalized to follow
+  Flutter's convention for component themes in the Material library. In 
+  `ThemeData`, the type of these properties have also changed accordingly.
+---
+
+## Summary
+
+`CardTheme`, `DialogTheme` and `TabBarTheme` were refactored to 
+conform to Flutter's conventions for component themes. `CardThemeData`, 
+`DialogThemeData` and `TabBarThemeData` were added to define overrides for the
+defaults of the component visual properties.
+
+During card theme normalization, the type of `ThemeData.cardTheme` is changed
+to `Object?` to accept both `CardTheme` and `CardThemeData`, in order to have
+a smooth transition for the breaking changes. The same approach was used for
+`dialogTheme` and `tabBarTheme`.
+
+To complete the transition and fully conform to the `ThemeData` convention, the 
+type of `ThemeData.cardTheme` has been changed to `CardThemeData?`; the type of
+`ThemeData.dialogTheme` has been changed to `DialogThemeData?`; and the type of
+`ThemeData.tabBarTheme` has been changed to `TabBarThemeData?`.
+
+## Migration guide
+
+Previously, the type of `ThemeData.cardTheme` was `Object?` to accpet both
+`CardTheme` and `CardThemeData`. Now that the type has been changed to
+`CardThemeData?`, a migration is required if `ThemeData.cardTheme` is used.
+Similarly, the types of `ThemeData.dialogTheme` and `ThemeData.tabBarTheme` 
+should be migrated to `DialogThemeData` and `TabBarThemeData`, respectively.
+
+Code before migration:
+
+```dart
+final ThemeData theme = ThemeData(
+    cardTheme: CardTheme(),
+    dialogTheme: DialogTheme(),
+    tabBarTheme: TabBarTheme(),
+);
+```
+
+Code after migration:
+
+```dart
+final ThemeData theme = ThemeData(
+    cardTheme: CardThemeData(),
+    dialogTheme: DialogThemeData(),
+    tabBarTheme: TabBarThemeData(),
+);
+```
+
+## Timeline
+
+Landed in version: TBD
+Stable release: TBD
+
+## References
+
+API documentation:
+
+* [`ThemeData`][]
+* [`CardTheme`][]
+* [`DialogTheme`][]
+* [`TabBarTheme`][]
+
+Relevant PRs:
+
+* [Change cardTheme, dialogTheme, and tabBarTheme type to xxxThemeData][]
+
+[Change cardTheme, dialogTheme, and tabBarTheme type to xxxThemeData]: {{site.github}}/flutter/flutter/pull/157292
+[`ThemeData`]: {{site.api}}/flutter/material/ThemeData-class.html
+[`CardTheme`]: {{site.api}}/flutter/material/CardTheme-class.html
+[`DialogTheme`]: {{site.api}}/flutter/material/DialogTheme-class.html
+[`TabBarTheme`]: {{site.api}}/flutter/material/TabBarTheme-class.html