tests/html: removed usage of deprecated unittest features

tests/html/xhr_cross_origin_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library XHRCrossOriginTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 import "dart:convert";
 
@@ skipping 72 matching lines @@
     test('XHR Cross-domain', () {
       var url =  '$host/root_dart/tests/html/xhr_cross_origin_data.txt';
       var xhr = new HttpRequest();
       xhr.open('GET', url, async: true);
       var validate = expectAsync((data) {
         expect(data, contains('feed'));
         expect(data['feed'], contains('entry'));
         expect(data, isMap);
       });
       xhr.onReadyStateChange.listen((e) {
-        guardAsync(() {
-          if (xhr.readyState == HttpRequest.DONE) {
-            validate(JSON.decode(xhr.response));
-          }
-        });
+        if (xhr.readyState == HttpRequest.DONE) {
+          validate(JSON.decode(xhr.response));
+        }
       });
       xhr.send();
     });
 
     test('XHR.getWithCredentials Cross-domain', () {
       var url = '$host/root_dart/tests/html/xhr_cross_origin_data.txt';
       return HttpRequest.request(url, withCredentials: true).then((xhr) {
         var data = JSON.decode(xhr.response);
         expect(data, contains('feed'));
         expect(data['feed'], contains('entry'));
         expect(data, isMap);
       });
     });
   });
 }