code: def __DeclareMessageAlias(self, schema, alias_for):
          
          # TODO(craigcitro): This is a hack. Remove it.
          message = extended_descriptor.ExtendedMessageDescriptor()
          message.name = self.__names.ClassName(schema['id'])
          message.alias_for = alias_for
          self.__DeclareDescriptor(message.name)
          self.__AddImport('from %s import extra_types' %
                           self.__base_files_package)
          self.__RegisterDescriptor(message)
docs: Declare schema as an alias for alias_for.