Adding conversion support for JSON XHR requests.

tools/dom/src/dart2js_Conversions.dart

Index: tools/dom/src/dart2js_Conversions.dart
diff --git a/tools/dom/src/dart2js_Conversions.dart b/tools/dom/src/dart2js_Conversions.dart
index 668ba3bec29a6ead8a00cfc81a0203014f2668b1..41b624e6c2e4be2e647bee146edf7983e9a68017 100644
--- a/tools/dom/src/dart2js_Conversions.dart
+++ b/tools/dom/src/dart2js_Conversions.dart
@@ -42,3 +42,10 @@ EventTarget _convertDartToNative_EventTarget(e) {
     return e;
   }
 }
+
+_convertNativeToDart_XHR_Response(o) {
+  if (o is Document) {
+    return o;
+  }
+  return convertNativeToDart_SerializedScriptValue(o);
+}