tribute(attribute, str);
        }
      }

      /**
       * Converts a typed JavaScript value to a string.
       *
       * This method is called by Polymer when setting JS property values to
       * HTML attributes.  Users may override this method on Polymer element
       * prototypes to provide serialization for custom types.
       *
       * @param {*} value Property value to serialize.
       * @return {string | undefined} String serialized from the provided property value.
       */
      _seria