Make the object store visible in Observatory.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index c6c61f5489af018ec0fcf5d1082aa9920dcab8c5..1a04111df38904f788366ad341f4272f267f1619 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1899,6 +1899,11 @@ void Isolate::PrintJSON(JSONStream* stream, bool ref) {
   }
 
   {
+    JSONObject objectStore(&jsobj, "_objectStore");
+    object_store()->PrintToJSONObject(&objectStore);
+  }
+
+  {
     JSONObject tagCounters(&jsobj, "_tagCounters");
     vm_tag_counters()->PrintToJSONObject(&tagCounters);
   }