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

runtime/vm/profiler.h

 // 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.
 
 #ifndef VM_PROFILER_H_
 #define VM_PROFILER_H_
 
 #include "vm/allocation.h"
 #include "vm/code_observers.h"
 #include "vm/globals.h"
@@ skipping 27 matching lines @@
   static void SetSampleDepth(intptr_t depth);
   static void SetSamplePeriod(intptr_t period);
 
   static void InitProfilingForIsolate(Isolate* isolate,
                                       bool shared_buffer = true);
   static void ShutdownProfilingForIsolate(Isolate* isolate);
 
   static void BeginExecution(Isolate* isolate);
   static void EndExecution(Isolate* isolate);
 
-  static void PrintToJSONStream(Isolate* isolate, JSONStream* stream,
-                                bool full, TagOrder tag_order);
+  static void PrintJSON(Isolate* isolate, JSONStream* stream,
+                        bool full, TagOrder tag_order);
   static void WriteProfile(Isolate* isolate);
 
   static SampleBuffer* sample_buffer() {
     return sample_buffer_;
   }
 
  private:
   static bool initialized_;
   static Monitor* monitor_;
 
@@ skipping 174 matching lines @@
   intptr_t capacity_;
   uintptr_t cursor_;
 
   DISALLOW_COPY_AND_ASSIGN(SampleBuffer);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_PROFILER_H_