update @immutable to support mixins

[pq]

With enhanced enums, it should be legal to annotate mixins.

For example:

@immutable
enum E {
  e(1), f(2), g(3);
  final int key;
  const E(this.key);
  bool operator ==(Object other) => other is E && other.key == key;
  int get hashCode => key.hashCode;
}

(Currently produces the diagnostic: "Only classes can be annotated as being immutable.")

See: #58586

[pq]

(whoops!)