only fired
      // from the magical button click submission.
      // See http://wayback.archive.org/web/20090323062817/http://blogs.vertigosoftware.com/snyholm/archive/2006/09/27/3788.aspx.
      var button = document.createElement('input');
      button.setAttribute('type', 'submit');
      form.appendChild(button);

      Polymer.clientSupportsFormValidationUI = true;
      form.addEventListener('submit', function(event) {
        // Oh good! We don't handle `required` correctly.
        Polymer.clientS