function createIframe(url = undefined) {
  const iframe = dom.createDom(TagName.IFRAME, {
    style: 'display: none',
    src: url || 'testdata/portchannel_inner.html',
  });

  return new GoogPromise((resolve, reject) => {
           events.listenOnce(iframe, EventType.LOAD, resolve);
           dom.appendChild(frameDiv, iframe);
         })
      .then((e) => iframe.contentWindow);
}
@param {string=} url A URL to use for the iframe src (defaults to
    "testdata/portchannel_inner.html").
@return {!GoogPromise<HTMLIFrameElement>} A promise that resolves with the
    loaded iframe.