mplate content to avoid the cost of cloning deeply nested templates.
       * This method retrieves the cached content for a given template.
       *
       * @param {HTMLTemplateElement} template Template to retrieve `content` for
       * @return {DocumentFragment} Content fragment
       */
      static _contentForTemplate(template) {
        let templateInfo = template.__templateInfo;
        return (templateInfo && templateInfo.content) || template.content;
      }

      /**
       * Clones the provid