[Isolates] Add a pointer to Heap to a meta map.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 35292271b6419e6a4f27dbcd8ed3c885615681ba..b77a95693e99424119e5c1fdb4d231c6eac9075f 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1121,6 +1121,8 @@ class Heap {
     return &external_string_table_;
   }
 
+  inline Isolate* isolate();
+
  private:
   Heap();