Relands #93452.

ChipThemeData is now conventional: all of its properties are default null and they define overrides of the defaults defined by the Chip widgets. Default overrides in priority order are (as usual):

• Chip widget constructor parameters
• A local ChipTheme
• The Theme's chipTheme

All of the ChipThemeData constructor parameters and properties are now nullable. Other than that and the change to the default value of ThemeData.chipTheme, this change is intended to be backwards compatible.

Addresses the "theme normalization" sub project within #91772

G3 Fix

This PR requires two small (Google internal) changes: cl/412111154

Breaking Change

There is a small chance that this PR will break existing apps which depend on the default value of the overall Theme's chipTheme.

The default value of Theme.of(context).chipTheme had been a ChipThemeData constructed like this:

chipTheme ??= ChipThemeData.fromDefaults(
  secondaryColor: isDark ? Colors.tealAccent[200]! : primaryColor,
  brightness: colorScheme.brightness,
  labelStyle: textTheme.bodyText1!,
);

This ensured that all of the properties of the chip theme returned by Theme.of(context).chipTheme were non-null. This rarely matters in application code and the defaults for all chip properties have not changed.

To remedy an app that has such a dependency, one can initialize the theme with the original default chipTheme value. For example:

MaterialApp(
  theme: ThemeData(
    primarySwatch: Colors.blue,
    chipTheme: ChipThemeData.fromDefaults(
      secondaryColor: Colors.blue,
      brightness: Brightness.light,
      labelStyle: Typography.material2014(
        platform: defaultTargetPlatform,
      ).white.bodyText1!
    ),
  ),
  ...
)

Generally speaking, it's preferable to use the theme's colorScheme and textTheme to lookup colors and text styles.