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

runtime/vm/coverage.h

Index: runtime/vm/coverage.h
diff --git a/runtime/vm/coverage.h b/runtime/vm/coverage.h
index d6680cf07786ac3c9a936e7ee18f1b9a8c3243cc..4e03c354f26312a072db7f2f4685db5c92a49f81 100644
--- a/runtime/vm/coverage.h
+++ b/runtime/vm/coverage.h
@@ -22,7 +22,7 @@ class JSONStream;
 class CodeCoverage : public AllStatic {
  public:
   static void Write(Isolate* isolate);
-  static void PrintToJSONStream(Isolate* isolate, JSONStream* stream);
+  static void PrintJSON(Isolate* isolate, JSONStream* stream);
 
  private:
   static void PrintClass(const Class& cls, const JSONArray& arr);