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

runtime/vm/coverage.cc

Index: runtime/vm/coverage.cc
diff --git a/runtime/vm/coverage.cc b/runtime/vm/coverage.cc
index b766c15636a95eff91856285e331b29184300505..c8007031e2c9bbd94a9cdd5bdf5bfd66e31b6541 100644
--- a/runtime/vm/coverage.cc
+++ b/runtime/vm/coverage.cc
@@ -156,7 +156,7 @@ void CodeCoverage::Write(Isolate* isolate) {
   }
 
   JSONStream stream;
-  PrintToJSONStream(isolate, &stream);
+  PrintJSON(isolate, &stream);
 
   const char* format = "%s/dart-cov-%" Pd "-%" Pd ".json";
   intptr_t pid = OS::ProcessId();
@@ -175,7 +175,7 @@ void CodeCoverage::Write(Isolate* isolate) {
 }
 
 
-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();