function keyFromUrlLike(url) {
  if (Array.isArray(url)) {
    return url.join('\n');
  }

  if (typeof url === 'string') {
    return url;
  }

  ++keyCount;
  return 'url-function-key-' + keyCount;
}
@param {UrlLike} url The URL-like option.
@return {string} A key for the URL.