property has a pending change.
       *
       * @param {string} prop Property name
       * @return {boolean} True if property has a pending change
       * @protected
       */
      _isPropertyPending(prop) {
        return this.__dataPending && (prop in this.__dataPending);
      }

      /**
       * Marks the properties as invalid, and enqueues an async
       * `_propertiesChanged` callback.
       *
       * @protected
       */
      _invalidateProperties() {
        if (!this.__dataInvalid && this