Add cross-origin test with credentials.

tests/html/html.status

 # 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.
 
 window_open_test: Skip # http://dartbug.com/5151
 
 event_test: Skip  # Issue 1996
 webgl_1_test: Skip  # Issue 1495
 xmldocument_test: Skip  # XML in dart:html is incomplete
 xmlelement_test: Skip  # XML in dart:html is incomplete
@@ skipping 254 matching lines @@
 input_element_test/supported_range: Fail
 # Interfaces not implemented: SVGTests, SVGLangSpace, SVGExternalResourcesRequired, SVGStylable
 svg_3_test: Fail
 svgelement_test/additionalConstructors: Fail
 svgelement2_test: Fail
 transferables_test: Fail   # Issue 3392.
 websql_test: Fail # FF does not support web SQL
 websocket_test: Pass, Fail # FF 10 is uses MozWebSocket, FF 11 uses WebSocket.
 # Exception... "Access to restricted URI denied" code: "1012" nsresult: "0x805303f4"
 #    (NS_ERROR_DOM_BAD_URI) ...
-xhr_test: Fail
 
 [ $runtime == ie9 && ($system == linux || $system == macos) ]
 *: Skip
 
 [ $runtime == safari && ($system == linux || $system == windows) ]
 *: Skip
 
 # 'html' tests import the HTML library, so they only make sense in
 # a browser environment.
 [ $runtime == vm ]
 *: Skip
 
 [ $compiler == dart2js ]
 transferables_test: Skip # until isInstance works in dart2js
 
 [ $compiler == dart2js && ($runtime == drt || $runtime == ff) ]
 request_animation_frame_test: Skip # Async test hangs.
 
 [ $compiler == dart2js && ($runtime == drt || $runtime == chrome || $runtime == ff || $runtime == safari) ]
 isolates_test: Skip   # Timeout because leg does not support web workers.
 typed_arrays_1_test: Fail # dart2js doesn't support is checks with generic types.
 
 [ $compiler == dart2js && $runtime == ff ]
-xhr_test: Skip # Timeout
 inner_frame_test: Skip # Timeout
 svg_3_test: Skip # Timeout
 documentfragment_test: Skip # Timeout