Not clear which non-widget data types get expanded and which don't


In this example, `Theme` has a `ThemeData`. It's not clear to me whether there's just a whitelisted handpicked set of "data" types that expands into its constituents and why `ThemeData` is opaque but `TextStyle` under `actionTextStyle` is expandable.

Also a bit inconsistent that `Theme` has an opaque `ThemeData` (which follows the real tree composition) but the `CupertinoTheme`'s `CupertinoThemeData` is somehow inlined into the `CupertinoTheme` node directly. Same with the `IconTheme` where the `IconThemeData` is inlined.