function getRequiredUrlParam(paramName, url = undefined) {
  return asserts.assertString(utils.getParamValue(
      url || util.getCurrentUrl(), paramName));
}
Gets the URL param identified by name from the given URL. If no URL is
provided, the one for the current page is used.
@param {string} paramName The name of the URL param.
@param {?string=} url The URL from which to extract the app ID.
@return {string} The param value.