code: def _MapItems(message, field):
      
      assert _IsMap(message, field)
      map_message = message.get_assigned_value(field.name)
      additional_properties = map_message.get_assigned_value(
          'additionalProperties')
      for kv_pair in additional_properties:
          yield kv_pair.key, kv_pair.value
docs: Yields the (key, value) pair of the map values.