hich serves as the binding scope for the template
       * instance the element is contained in.  A template model should be used
       * to manipulate data associated with this template instance.
       *
       * @param {HTMLElement} el Element for which to return a template model.
       * @return {TemplateInstanceBase} Model representing the binding scope for
       *   the element.
       */
      modelForElement(el) {
        return Polymer.Templatize.modelForElement(this._templatizerTemplate, el);
 