Add option to get only reference.

runtime/vm/service.cc

Index: runtime/vm/service.cc
===================================================================
--- runtime/vm/service.cc	(revision 42442)
+++ runtime/vm/service.cc	(working copy)
@@ -2357,6 +2357,7 @@
     PrintError(js, "Must specify address: address/" Px ".", kExampleAddr);
     return true;
   }
+  bool ref = js->HasOption("ref") && js->OptionIs("ref", "true");
   Object& object = Object::Handle(isolate);
   {
     NoGCScope no_gc;
@@ -2363,7 +2364,7 @@
     ContainsAddressVisitor visitor(isolate, addr);
     object = isolate->heap()->FindObject(&visitor);
   }
-  object.PrintJSON(js, false);
+  object.PrintJSON(js, ref);
   return true;
 }