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

runtime/vm/class_table.h

 // 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.
 
 #ifndef VM_CLASS_TABLE_H_
 #define VM_CLASS_TABLE_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 
@@ skipping 140 matching lines @@
   uword PredefinedClassHeapStatsTableAddress() {
     return reinterpret_cast<uword>(predefined_class_heap_stats_table_);
   }
 
   // Used by generated code.
   uword ClassStatsTableAddress() {
     return reinterpret_cast<uword>(&class_heap_stats_table_);
   }
 
 
-  void AllocationProfilePrintToJSONStream(JSONStream* stream);
+  void AllocationProfilePrintJSON(JSONStream* stream);
   void ResetAllocationAccumulators();
 
  private:
   friend class MarkingVisitor;
   friend class ScavengerVisitor;
   friend class ClassHeapStatsTestHelper;
   static const int initial_capacity_ = 512;
   static const int capacity_increment_ = 256;
 
   static bool ShouldUpdateSizeForClassId(intptr_t cid);
 
   intptr_t top_;
   intptr_t capacity_;
 
   RawClass** table_;
   ClassHeapStats* class_heap_stats_table_;
 
   ClassHeapStats* predefined_class_heap_stats_table_;
 
   ClassHeapStats* StatsAt(intptr_t cid);
   void UpdateLiveOld(intptr_t cid, intptr_t size);
   void UpdateLiveNew(intptr_t cid, intptr_t size);
 
   DISALLOW_COPY_AND_ASSIGN(ClassTable);
 };
 
 }  // namespace dart
 
 #endif  // VM_CLASS_TABLE_H_