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

runtime/vm/debugger.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/debugger.h"
 
 #include "include/dart_api.h"
 
 #include "vm/code_generator.h"
 #include "vm/code_patcher.h"
@@ skipping 117 matching lines @@
   return line_number_;
 }
 
 
 void SourceBreakpoint::VisitObjectPointers(ObjectPointerVisitor* visitor) {
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&script_));
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&function_));
 }
 
 
-void SourceBreakpoint::PrintToJSONStream(JSONStream* stream) {
+void SourceBreakpoint::PrintJSON(JSONStream* stream) {
   Isolate* isolate = Isolate::Current();
 
   JSONObject jsobj(stream);
   jsobj.AddProperty("type", "Breakpoint");
 
   jsobj.AddProperty("id", id());
   jsobj.AddProperty("enabled", IsEnabled());
   jsobj.AddProperty("resolved", IsResolved());
 
   Library& library = Library::Handle(isolate);
@@ skipping 2365 matching lines @@
 }
 
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 }  // namespace dart