As part of the Material 3 Migration, this PR adds support for the new surface tint overlay feature to the Material widget. If a new surfaceTintColor is provided to the Material widget it will apply this color to the base color with an opacity based on the elevation. This is described in detail in the Material 3 spec.

In order to use this new feature you must opt-in with the useMaterial3 flag:

  return MaterialApp(
    theme: ThemeData(useMaterial3: true),
    // ...
  );

Then you can use the new surfaceTintColor to turn on an overlay for the given Material widget:

   Material(
     elevation: 6,
     surfaceTintColor: Theme.of(context).colorScheme.primary,
     shadowColor: Theme.of(context).colorScheme.shadow,
   );

This PR also changes the default value for Material.shadowColor when ThemeData.useMaterial3 is true. Previous to Material 3, if shadowColor was null it would default to ThemeData.shadowColor. For Material 3 in some situations we need to be able to turn off the drop shadow even if there is an elevation. Therefore if ThemeData.useMaterial3 is true and shadowColor is null then no drop shadow will be rendered.

Existing M2 tests were moved to their own group and new M3 tests were added.

Fixes: #91607

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.