Make sure transitioned arrays efficiently call builtin Array functions

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 9aee6790ceec43e2606d2b65a6d3dd20b3d5ba4f..66cb699fcd7530373e861f271b4e5b12d5b0bcfd 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -1,4 +1,4 @@
-// Copyright 2011 the V8 project authors. All rights reserved.
+// Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
@@ -2363,7 +2363,9 @@ Object* Map::GetDescriptorContents(String* sentinel_name,
     if (details.type() == ELEMENTS_TRANSITION) {
       return descriptors->GetValue(index);
     } else {
-      *safe_to_add_transition = false;
+      if (safe_to_add_transition != NULL) {
+        *safe_to_add_transition = false;
+      }
     }
   }
   return NULL;