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

runtime/vm/profiler.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/utils.h"
 
 #include "vm/allocation.h"
 #include "vm/atomic.h"
 #include "vm/code_patcher.h"
 #include "vm/isolate.h"
@@ skipping 1309 matching lines @@
 
   CodeRegionTrieNode* exclusive_root_;
   CodeRegionTable* live_code_table_;
   CodeRegionTable* dead_code_table_;
   CodeRegionTable* tag_code_table_;
   intptr_t dead_code_table_offset_;
   intptr_t tag_code_table_offset_;
 };
 
 
-void Profiler::PrintToJSONStream(Isolate* isolate, JSONStream* stream,
-                                 bool full, TagOrder tag_order) {
+void Profiler::PrintJSON(Isolate* isolate, JSONStream* stream,
+                         bool full, TagOrder tag_order) {
   ASSERT(isolate == Isolate::Current());
   // Disable profile interrupts while processing the buffer.
   EndExecution(isolate);
   MutexLocker profiler_data_lock(isolate->profiler_data_mutex());
   IsolateProfilerData* profiler_data = isolate->profiler_data();
   if (profiler_data == NULL) {
     JSONObject error(stream);
     error.AddProperty("type", "Error");
     error.AddProperty("text", "Isolate does not have profiling enabled.");
     return;
@@ skipping 114 matching lines @@
   Dart_FileCloseCallback file_close = Isolate::file_close_callback();
   Dart_FileWriteCallback file_write = Isolate::file_write_callback();
   if ((file_open == NULL) || (file_close == NULL) || (file_write == NULL)) {
     // Embedder has not provided necessary callbacks.
     return;
   }
   // We will be looking up code objects within the isolate.
   ASSERT(Isolate::Current() == isolate);
   JSONStream stream(10 * MB);
   intptr_t pid = OS::ProcessId();
-  PrintToJSONStream(isolate, &stream, true, Profiler::kNoTags);
+  PrintJSON(isolate, &stream, true, Profiler::kNoTags);
   const char* format = "%s/dart-profile-%" Pd "-%" Pd ".json";
   intptr_t len = OS::SNPrint(NULL, 0, format,
                              FLAG_profile_dir, pid, isolate->main_port());
   char* filename = Isolate::Current()->current_zone()->Alloc<char>(len + 1);
   OS::SNPrint(filename, len + 1, format,
               FLAG_profile_dir, pid, isolate->main_port());
   void* f = file_open(filename, true);
   if (f == NULL) {
     // Cannot write.
     return;
@@ skipping 246 matching lines @@
       ProfilerDartStackWalker stackWalker(sample);
       stackWalker.walk();
     } else {
       // TODO(johnmccutchan): Support collecting only Dart frames with
       // ProfilerNativeStackWalker.
     }
   }
 }
 
 }  // namespace dart