Following https://github.com/flutter/flutter/pull/153982, this PR is to normalize `ThemeData.dialogTheme`; change the `DialogTheme dialogTheme` property to `DialogThemeData dialogTheme` in ThemeData. In `ThemeData()` and `ThemeData.copyWith()`, the dialogTheme parameter type is changed to `Object?` to accept both `DialogTheme` and `DialogThemeData` 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 `DialogTheme`; `DialogTheme` is a new `InheritedWidget` subclass now.
Addresses the "theme normalization" sub project within https://github.com/flutter/flutter/issues/91772
64906c28a6