in-turn backstop to prevent
     * cycles due to 2-way notification).
     *
     * @param {string} property Property name
     * @param {*} value New property value
     * @param {*} old Previous property value
     * @return {boolean} Whether the property should be considered a change
     * @protected
     */
    _shouldPropertyChange(property, value, old) {
      return mutablePropertyChange(this, property, value, old, true);
    }
  };

  /**
   * Legacy element behavior to add the optional ability to 