code: function (url, options, cb) {
          if (isFunction(options)) {
              const t = cb;
              cb = options;
              options = t;
          }
          if (IS_NODE && Ajax.get.node) {
              return Ajax.get.node(url, cb, options);
          }
          const client = Ajax._getClient(cb);
          client.open('GET', url, true);
          if (options) {
              for (const k in options.headers) {
                  client.setRequestHeader(k, options.headers[k]);
              }
              client.withCredentials = options.credentials === 'include';
              if (options['responseType']) {
                  client.responseType = options['responseType'];
              }
          }
          client.send(null);
          return client;
      }
docs: Fetch remote resource by HTTP "GET" method
  @param  {String}   url - resource url
  @param  {Object}   [options=null] - request options
  @param  {Object}   [options.headers=null] - HTTP headers
  @param  {String}   [options.responseType=null] - responseType
  @param  {String}   [options.credentials=null]  - if with credentials, set it to "include"
  @param  {Function} cb  - callback function when completed
  @return {Ajax}  Ajax
  @example
  maptalks.Ajax.get(
  'url/to/resource',
  (err, data) => {
  if (err) {
  throw new Error(err);
  }
  // do things with data
  }
  );