wallet-core/thirdparty/systemjs/lib/system-fetch.js

  var fetchTextFromURL;
  if (typeof XMLHttpRequest != 'undefined') {
    fetchTextFromURL = function(url, authorization, fulfill, reject) {
      var xhr = new XMLHttpRequest();
      var sameDomain = true;
      var doTimeout = false;
      if (!('withCredentials' in xhr)) {
        // check if same domain
        var domainCheck = /^(\w+:)?\/\/([^\/]+)/.exec(url);
        if (domainCheck) {
          sameDomain = domainCheck[2] === window.location.host;
          if (domainCheck[1])
            sameDomain &= domainCheck[1] === window.location.protocol;
        }
      }
      if (!sameDomain && typeof XDomainRequest != 'undefined') {
        xhr = new XDomainRequest();
        xhr.onload = load;
        xhr.onerror = error;
        xhr.ontimeout = error;
        xhr.onprogress = function() {};
        xhr.timeout = 0;
        doTimeout = true;
      }
      function load() {
        fulfill(xhr.responseText);
      }
      function error() {
        reject(new Error('XHR error' + (xhr.status ? ' (' + xhr.status + (xhr.statusText ? ' ' + xhr.statusText  : '') + ')' : '') + ' loading ' + url));
      }

      xhr.onreadystatechange = function () {
        if (xhr.readyState === 4) {
          // in Chrome on file:/// URLs, status is 0
          if (xhr.status == 0) {
            if (xhr.responseText) {
              load();
            }
            else {
              // when responseText is empty, wait for load or error event
              // to inform if it is a 404 or empty file
              xhr.addEventListener('error', error);
              xhr.addEventListener('load', load);
            }
          }
          else if (xhr.status === 200) {
            load();
          }
          else {
            error();
          }
        }
      };
      xhr.open("GET", url, true);

      if (xhr.setRequestHeader) {
        xhr.setRequestHeader('Accept', 'application/x-es-module, */*');
        // can set "authorization: true" to enable withCredentials only
        if (authorization) {
          if (typeof authorization == 'string')
            xhr.setRequestHeader('Authorization', authorization);
          xhr.withCredentials = true;
        }
      }

      if (doTimeout) {
        setTimeout(function() {
          xhr.send();
        }, 0);
      } else {
        xhr.send(null);
      }
    };
  }
  else if (typeof require != 'undefined' && typeof process != 'undefined') {
    var fs;
    fetchTextFromURL = function(url, authorization, fulfill, reject) {
      if (url.substr(0, 8) != 'file:///')
        throw new Error('Unable to fetch "' + url + '". Only file URLs of the form file:/// allowed running in Node.');
      fs = fs || require('fs');
      if (isWindows)
        url = url.replace(/\//g, '\\').substr(8);
      else
        url = url.substr(7);
      return fs.readFile(url, function(err, data) {
        if (err) {
          return reject(err);
        }
        else {
          // Strip Byte Order Mark out if it's the leading char
          var dataString = data + '';
          if (dataString[0] === '\ufeff')
            dataString = dataString.substr(1);

          fulfill(dataString);
        }
      });
    };
  }
  else if (typeof self != 'undefined' && typeof self.fetch != 'undefined') {
    fetchTextFromURL = function(url, authorization, fulfill, reject) {
      var opts = {
        headers: {'Accept': 'application/x-es-module, */*'}
      };

      if (authorization) {
        if (typeof authorization == 'string')
          opts.headers['Authorization'] = authorization;
        opts.credentials = 'include';
      }

      fetch(url, opts)
        .then(function (r) {
          if (r.ok) {
            return r.text();
          } else {
            throw new Error('Fetch error: ' + r.status + ' ' + r.statusText);
          }
        })
        .then(fulfill, reject);
    }
  }
  else {
    throw new TypeError('No environment fetch API available.');
  }