Material Theme System Updates

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?.

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:

final ThemeData theme = ThemeData(
    cardTheme: CardTheme(),
    dialogTheme: DialogTheme(),
    tabBarTheme: TabBarTheme(),
);

Code after migration:

final ThemeData theme = ThemeData(
    cardTheme: CardThemeData(),
    dialogTheme: DialogThemeData(),
    tabBarTheme: TabBarThemeData(),
);

Landed in version: TBD Stable release: TBD

API documentation:

• ThemeData
• CardTheme
• DialogTheme
• TabBarTheme

Relevant PRs:

• Change cardTheme, dialogTheme, and tabBarTheme type to xxxThemeData

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