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

runtime/vm/class_table.cc

 // 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.
 
 #include "vm/class_table.h"
 #include "vm/flags.h"
 #include "vm/freelist.h"
 #include "vm/heap.h"
 #include "vm/object.h"
 #include "vm/raw_object.h"
@@ skipping 307 matching lines @@
 void ClassTable::ResetCountersNew() {
   for (intptr_t i = 0; i < kNumPredefinedCids; i++) {
     predefined_class_heap_stats_table_[i].ResetAtNewGC();
   }
   for (intptr_t i = kNumPredefinedCids; i < top_; i++) {
     class_heap_stats_table_[i].ResetAtNewGC();
   }
 }
 
 
-void ClassTable::AllocationProfilePrintToJSONStream(JSONStream* stream) {
+void ClassTable::AllocationProfilePrintJSON(JSONStream* stream) {
   Isolate* isolate = Isolate::Current();
   ASSERT(isolate != NULL);
   Heap* heap = isolate->heap();
   ASSERT(heap != NULL);
   JSONObject obj(stream);
   obj.AddProperty("type", "AllocationProfile");
   obj.AddProperty("id", "allocationprofile");
   {
     JSONObject heaps(&obj, "heaps");
     {
@@ skipping 87 matching lines @@
 
 void ClassTable::UpdateLiveNew(intptr_t cid, intptr_t size) {
   ClassHeapStats* stats = StatsAt(cid);
   ASSERT(stats != NULL);
   ASSERT(size >= 0);
   stats->post_gc.AddNew(size);
 }
 
 
 }  // namespace dart