XTypeGroup should be marked as @immutable, and its constructor made const, so mutable fields aren't accidentally added to it.

This will require a three-step process because of our analysis settings:

• Annotate all creation of XTypeGroups in the plugin's other package's with // ignore: prefer_const_constructors.
• Make the change in the platform interface package.
• Update the other packages to require that new version of the platform interface package, remove the ignore's and add the consts.