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

runtime/vm/coverage.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 "vm/coverage.h"
 
 #include "include/dart_api.h"
 
 #include "vm/compiler.h"
 #include "vm/isolate.h"
@@ skipping 138 matching lines @@
   }
 
   Dart_FileOpenCallback file_open = Isolate::file_open_callback();
   Dart_FileWriteCallback file_write = Isolate::file_write_callback();
   Dart_FileCloseCallback file_close = Isolate::file_close_callback();
   if ((file_open == NULL) || (file_write == NULL) || (file_close == NULL)) {
     return;
   }
 
   JSONStream stream;
-  PrintToJSONStream(isolate, &stream);
+  PrintJSON(isolate, &stream);
 
   const char* format = "%s/dart-cov-%" Pd "-%" Pd ".json";
   intptr_t pid = OS::ProcessId();
   intptr_t len = OS::SNPrint(NULL, 0, format,
                              FLAG_coverage_dir, pid, isolate->main_port());
   char* filename = Isolate::Current()->current_zone()->Alloc<char>(len + 1);
   OS::SNPrint(filename, len + 1, format,
               FLAG_coverage_dir, pid, isolate->main_port());
   void* file = (*file_open)(filename, true);
   if (file == NULL) {
     OS::Print("Failed to write coverage file: %s\n", filename);
     return;
   }
   (*file_write)(stream.buffer()->buf(), stream.buffer()->length(), file);
   (*file_close)(file);
 }
 
 
-void CodeCoverage::PrintToJSONStream(Isolate* isolate, JSONStream* stream) {
+void CodeCoverage::PrintJSON(Isolate* isolate, JSONStream* stream) {
   const GrowableObjectArray& libs = GrowableObjectArray::Handle(
       isolate, isolate->object_store()->libraries());
   Library& lib = Library::Handle();
   Class& cls = Class::Handle();
   JSONObject coverage(stream);
   coverage.AddProperty("type", "CodeCoverage");
   coverage.AddProperty("id", "coverage");
   {
     JSONArray jsarr(&coverage, "coverage");
     for (int i = 0; i < libs.Length(); i++) {
       lib ^= libs.At(i);
       ClassDictionaryIterator it(lib, ClassDictionaryIterator::kIteratePrivate);
       while (it.HasNext()) {
         cls = it.GetNextClass();
         if (cls.EnsureIsFinalized(isolate) == Error::null()) {
           // Only classes that have been finalized do have a meaningful list of
           // functions.
           PrintClass(cls, jsarr);
         }
       }
     }
   }
 }
 
 
 }  // namespace dart