Add infrastructure to keep track of references to prototypes.

src/lookup.cc

Index: src/lookup.cc
diff --git a/src/lookup.cc b/src/lookup.cc
index 55e9d689a118765c97256aee5d5f98a8138d3ce2..142d9ee70070474ab37aa8d4d8b69b287a0e53e4 100644
--- a/src/lookup.cc
+++ b/src/lookup.cc
@@ -127,7 +127,7 @@ void LookupIterator::PrepareTransitionToDataProperty(
   // observable.
   Handle<JSObject> receiver = GetStoreTarget();
 
-  if (!name().is_identical_to(isolate()->factory()->hidden_string()) &&
+  if (!isolate()->IsInternallyUsedPropertyName(name()) &&
       !receiver->map()->is_extensible()) {
     return;
   }