Add cross-origin test with credentials.

tests/html/form_data_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 FormDataTest;
 
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
@@ skipping 41 matching lines @@
 
   test('send', () {
     var form = new FormData();
     var blobString = 'Indescribable... Indestructible! Nothing can stop it!';
     var blob = new Blob(
         [blobString],
         'text/plain');
     form.append('theBlob', blob, 'theBlob.txt');
 
     var xhr = new HttpRequest();
-    xhr.open("POST", "http://localhost:9876/echo");
+    xhr.open("POST", "http://localhost:${window.location.port}/echo");
 
     xhr.on.load.add(expectAsync1((e) {
       expect(xhr.responseText.contains(blobString), true);
     }));
     xhr.on.error.add((e) {
       fail('$e');
     });
     xhr.send(form);
   });
 }