const req = new XMLHttpRequest();
         req.open('get', '/', true);
         const listener = req.onreadystatechange = function() {
           if (req.readyState === 4) {
             done();
           }
         };
         expect(req.onreadystatechange).toBe(listener);
         req.onreadystatechange = function() {
           return listener.apply(this, arguments);
         };
         req.send();