function assertFederatedLinkingPage(opt_email) {
  assertPage_(container, 'firebaseui-id-page-federated-linking');
  if (opt_email) {
    assertPageContainsText(opt_email);
  }
}
Asserts federated linking page is displayed. If an email is provided, it
checks that the email is populated in the page body.
@param {string=} opt_email Optional email to check.