o start loading.
   * In the `onload` callback, the `import` property of the `link`
   * element will contain the imported document contents.
   *
   * @memberof Polymer
   * @param {string} href URL to document to load.
   * @param {?function(!Event):void=} onload Callback to notify when an import successfully
   *   loaded.
   * @param {?function(!ErrorEvent):void=} onerror Callback to notify when an import
   *   unsuccessfully loaded.
   * @param {boolean=} optAsync True if the import should be loaded `