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

runtime/vm/debugger.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_DEBUGGER_H_
 #define VM_DEBUGGER_H_
 
 #include "include/dart_debugger_api.h"
 
 #include "vm/object.h"
@@ skipping 30 matching lines @@
   RawString* SourceUrl();
   intptr_t LineNumber();
 
   void GetCodeLocation(Library* lib, Script* script, intptr_t* token_pos);
 
   void Enable();
   void Disable();
   bool IsEnabled() const { return is_enabled_; }
   bool IsResolved() { return is_resolved_; }
 
-  void PrintToJSONStream(JSONStream* stream);
+  void PrintJSON(JSONStream* stream);
 
  private:
   void VisitObjectPointers(ObjectPointerVisitor* visitor);
 
   void SetResolved(const Function& func, intptr_t token_pos);
   void set_next(SourceBreakpoint* value) { next_ = value; }
   SourceBreakpoint* next() const { return this->next_; }
 
   const intptr_t id_;
   RawScript* script_;
@@ skipping 439 matching lines @@
 
   friend class Isolate;
   friend class SourceBreakpoint;
   DISALLOW_COPY_AND_ASSIGN(Debugger);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_DEBUGGER_H_