async`.
   *   Defaults to `false`.
   * @return {!HTMLLinkElement} The link element for the URL to be loaded.
   */
  Polymer.importHref = function(href, onload, onerror, optAsync) {
    let link = /** @type {HTMLLinkElement} */
      (document.head.querySelector('link[href="' + href + '"][import-href]'));
    if (!link) {
      link = /** @type {HTMLLinkElement} */ (document.createElement('link'));
      link.rel = 'import';
      link.href = href;
      link.setAttribute('import-href', '');
    }
    // 