Adding conversion support for JSON XHR requests.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 9d221b16bd2d08ad8a1280257e08d23a4fdc71af..df4deec3c1841bd8001cfca8f78f338155d5afa5 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -13099,6 +13099,8 @@ class HttpRequest extends XmlHttpRequestEventTarget native "XMLHttpRequest" {
   @DocsEditable()
   final int readyState;
 
+  dynamic get response => _convertNativeToDart_XHR_Response(this._get_response);
+  @JSName('response')
   /**
    * The data received as a reponse from the request.
    *
@@ -13113,7 +13115,7 @@ class HttpRequest extends XmlHttpRequestEventTarget native "XMLHttpRequest" {
   @SupportedBrowser(SupportedBrowser.IE, '10')
   @SupportedBrowser(SupportedBrowser.SAFARI)
   @Creates('ByteBuffer|Blob|Document|=Object|JSExtendableArray|String|num')
-  final Object response;
+  final dynamic _get_response;
 
   /**
    * The response in string form or `null on failure.
@@ -31837,6 +31839,13 @@ EventTarget _convertDartToNative_EventTarget(e) {
     return e;
   }
 }
+
+_convertNativeToDart_XHR_Response(o) {
+  if (o is Document) {
+    return o;
+  }
+  return convertNativeToDart_SerializedScriptValue(o);
+}
 // Copyright (c) 2013, 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.