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

runtime/vm/coverage.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_COVERAGE_H_
 #define VM_COVERAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/flags.h"
 
 namespace dart {
 
 DECLARE_FLAG(charp, coverage_dir);
 
 // Forward declarations.
 class Class;
 class Function;
 class Isolate;
 class JSONArray;
 class JSONStream;
 
 class CodeCoverage : public AllStatic {
  public:
   static void Write(Isolate* isolate);
-  static void PrintToJSONStream(Isolate* isolate, JSONStream* stream);
+  static void PrintJSON(Isolate* isolate, JSONStream* stream);
 
  private:
   static void PrintClass(const Class& cls, const JSONArray& arr);
   static void CompileAndAdd(const Function& function,
                             const JSONArray& hits_arr);
 };
 
 }  // namespace dart
 
 #endif  // VM_COVERAGE_H_