function createDialog(opt_content) {
    var dialog = document.createElement('dialog');
    dialog.innerHTML = opt_content || 'Dialog #' + (cleanup.length);
    document.body.appendChild(dialog);
    if (window.location.search == '?force') {
      dialogPolyfill.forceRegisterDialog(dialog);
    } else {
      dialogPolyfill.registerDialog(dialog);
    }
    return cleanup(dialog);
  }
Creates a dialog for testing that will be cleaned up later.

@param {string?} opt_content to be used as innerHTML