function assertCallbackPageDomHidden(container) {
  const element = dom.getElementByClass(
      'firebaseui-id-page-callback', container);
  assertNotNull(element);
  assertTrue(dom.classlist.contains(element, 'firebaseui-hidden'));
}
Asserts the callback page is in the dom but hidden.
@param {!Element} container The html element container of the widget.