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

runtime/vm/json_stream.cc

 // 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.
 
 #include "platform/assert.h"
 
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/json_stream.h"
 #include "vm/message.h"
@@ skipping 219 matching lines @@
   ASSERT(len == len2);
   buffer_.AddChar('"');
   AddEscapedUTF8String(p);
   buffer_.AddChar('"');
   free(p);
 }
 
 
 void JSONStream::PrintValue(const Object& o, bool ref) {
   PrintCommaIfNeeded();
-  o.PrintToJSONStream(this, ref);
+  o.PrintJSON(this, ref);
 }
 
 
 void JSONStream::PrintValue(SourceBreakpoint* bpt) {
   PrintCommaIfNeeded();
-  bpt->PrintToJSONStream(this);
+  bpt->PrintJSON(this);
 }
 
 
 void JSONStream::PrintValue(Isolate* isolate, bool ref) {
   PrintCommaIfNeeded();
-  isolate->PrintToJSONStream(this, ref);
+  isolate->PrintJSON(this, ref);
 }
 
 
 void JSONStream::PrintPropertyBool(const char* name, bool b) {
   PrintPropertyName(name);
   PrintValueBool(b);
 }
 
 
 void JSONStream::PrintProperty(const char* name, intptr_t i) {
@@ skipping 148 matching lines @@
   intptr_t len2 = OS::VSNPrint(p, len+1, format, args);
   va_end(args);
   ASSERT(len == len2);
   stream_->buffer_.AddChar('"');
   stream_->AddEscapedUTF8String(p);
   stream_->buffer_.AddChar('"');
   free(p);
 }
 
 }  // namespace dart