Construct a unique union type name based on the type names being unioned.

def _get_union_type_name(type_names_to_union):
    """Construct a unique union type name based on the type names being unioned."""
    if not type_names_to_union:
        raise AssertionError(u'Expected a non-empty list of type names to union, received: '
                             u'{}'.format(type_names_to_union))
    return u'Union__' + u'__'.join(sorted(type_names_to_union))