tests/html: removed usage of deprecated unittest features

tests/html/url_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 url_test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 import 'dart:typed_data';
 
@@ skipping 28 matching lines @@
       var blob = createImageBlob();
       var url = Url.createObjectUrlFromBlob(blob);
       expect(url.length, greaterThan(0));
       expect(url, startsWith('blob:'));
 
       var img = new ImageElement();
       img.onLoad.listen(expectAsync((_) {
         expect(img.complete, true);
       }));
       img.onError.listen((_) {
-        guardAsync(() {
-          expect(true, isFalse, reason: 'URL failed to load.');
-        });
+        fail('URL failed to load.');
       });
       img.src = url;
     });
 
     test('revokeObjectUrl', () {
       var blob = createImageBlob();
       var url = Url.createObjectUrlFromBlob(blob);
       expect(url, startsWith('blob:'));
       Url.revokeObjectUrl(url);
 
       var img = new ImageElement();
       // Image should fail to load since the URL was revoked.
       img.onError.listen(expectAsync((_) {
       }));
       img.onLoad.listen((_) {
-        guardAsync(() {
-          expect(true, isFalse, reason: 'URL should not have loaded.');
-        });
+        fail('URL should not have loaded.');
       });
       img.src = url;
     });
 
   });
 }