e created.
       *
       * The default implementation sets these properties back into the
       * setter at instance time.  This method is provided as an override
       * point for customizing or providing more efficient initialization.
       *
       * @param {Object} props Bag of property values that were overwritten
       *   when creating property accessors.
       * @protected
       */
      _initializeProtoProperties(props) {
        for (let p in props) {
          this._setProperty(p, props[p