Read isolate from page, not through map.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 996a18657af48db72a38c0d2b9470fe6c63a9e2e..d08075464e04c7f216735b9836e80895842c0c49 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1,4 +1,4 @@
-// Copyright 2010 the V8 project authors. All rights reserved.
+// Copyright 2011 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:
@@ -3725,9 +3725,6 @@ class Map: public HeapObject {
   inline int visitor_id();
   inline void set_visitor_id(int visitor_id);
 
-  // Returns the heap this map belongs to.
-  inline Heap* heap();
-
   typedef void (*TraverseCallback)(Map* map, void* data);
 
   void TraverseTransitionTree(TraverseCallback callback, void* data);