code: function buildAbsoluteUrl(url, protocol, host) {
      if (protocolRelativeRegex.test(url)) {
        url = host + url;
      } else if (!httpRegex.test(url)) {
        if (!host) {
          throw new Error(
            'You are using using fetch with a path-relative URL, but host is missing from Fastboot request. Please set the hostWhitelist property in your environment.js.'
          );
        }
        url = protocol + '//' + host + url;
      }
      return url;
    }
docs: Build the absolute url if it's not, can handle:
  - protocol-relative URL (//can-be-http-or-https.com/)
  - path-relative URL (/file/under/root)
  
  @param {string} url
  @param {string} protocol
  @param {string} host
  @returns {string}