.toString() : undefined;
        }
      }

      /**
       * Converts a string to a typed JavaScript value.
       *
       * This method is called by Polymer when reading HTML attribute values to
       * JS properties.  Users may override this method on Polymer element
       * prototypes to provide deserialization for custom `type`s.  Note,
       * the `type` argument is the value of the `type` field provided in the
       * `properties` configuration object for a given property, and is
       * by co