Adding conversion support for JSON XHR requests.

tools/dom/src/dart2js_Conversions.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.
 
 
 // Conversions for Window.  These check if the window is the local
 // window, and if it's not, wraps or unwraps it with a secure wrapper.
 // We need to test for EventTarget here as well as it's a base type.
 // We omit an unwrapper for Window as no methods take a non-local
 // window as a parameter.
@@ skipping 24 matching lines @@
     return e;
 }
 
 EventTarget _convertDartToNative_EventTarget(e) {
   if (e is _DOMWindowCrossFrame) {
     return e._window;
   } else {
     return e;
   }
 }
+
+_convertNativeToDart_XHR_Response(o) {
+  if (o is Document) {
+    return o;
+  }
+  return convertNativeToDart_SerializedScriptValue(o);
+}