Always use the same json for null in the vm service.

runtime/vm/isolate.cc

 // 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.
 
 #include "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "lib/mirrors.h"
@@ skipping 864 matching lines @@
 }
 
 
 void Isolate::VisitPrologueWeakPersistentHandles(HandleVisitor* visitor) {
   if (api_state() != NULL) {
     api_state()->VisitPrologueWeakHandles(visitor);
   }
 }
 
 
-void Isolate::PrintToJSONStream(JSONStream* stream, bool ref) {
+void Isolate::PrintJSON(JSONStream* stream, bool ref) {
   JSONObject jsobj(stream);
   jsobj.AddProperty("type", (ref ? "@Isolate" : "Isolate"));
   jsobj.AddPropertyF("id", "isolates/%" Pd "",
                      static_cast<intptr_t>(main_port()));
   jsobj.AddPropertyF("mainPort", "%" Pd "",
                      static_cast<intptr_t>(main_port()));
 
   // Assign an isolate name based on the entry function.
   IsolateSpawnState* state = spawn_state();
   if (state == NULL) {
@@ skipping 271 matching lines @@
   return func.raw();
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(isolate());
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart