
Following https://github.com/flutter/flutter/pull/155476, this PR is to normalize `ThemeData.tabBarTheme`; change the `TabBarTheme tabBarTheme` property to `TabBarThemeData tabBarTheme` in `ThemeData`. In `ThemeData()` and `ThemeData.copyWith()`, the `tabBarTheme` parameter type is changed to `Object?` to accept both `TabBarTheme` and `TabBarThemeData` so that we won't cause immediate breaking change and make sure rolling is smooth. Once all component themes are normalized, these `Object?` types should be changed to `xxxThemeData`. There's no way to create a dart fix because we can't add a "@deprecated" label for TabBarTheme; TabBarTheme is a new InheritedWidget subclass now. Addresses the "theme normalization" sub project within https://github.com/flutter/flutter/issues/91772
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