A dummy superclass to support GWT serialization of the element type of an ImmutableMultiset. The GWT supersource for this class contains a field of type E.

For details about this hack, see GwtSerializationDependencies, which takes the same approach but with a subclass rather than a superclass.

TODO(cpovirk): Consider applying this subclass approach to our other types.

For ImmutableMultiset in particular, I ran into a problem with the GwtSerializationDependencies approach: When autogenerating a serializer for the new class, GWT tries to refer to our dummy serializer for the superclass, ImmutableMultiset_CustomFieldSerializer. But that type has no methods (since it's never actually used). We could probably fix the problem by adding dummy methods to that class, but that is starting to sound harder than taking the superclass approach, which I've been coming to like, anyway, since it doesn't require us to declare dummy methods (though occasionally constructors) and make types non-final.