code: @GwtCompatible(serializable = true)
    public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(
        E anElement, E... otherElements) {
      return ImmutableEnumSet.asImmutable(EnumSet.of(anElement, otherElements));
    }
docs: http://code.google.com/p/google-web-toolkit/issues/detail?id=3028