Faster moving FixedArray elements around.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 318aac32823879750cc16df6e32d1d07792cf6f7..2494f9bc6957291afa2b39a1287e75d0893e75de 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -771,6 +771,9 @@ class Heap : public AllStatic {
   // Write barrier support for address[offset] = o.
   static inline void RecordWrite(Address address, int offset);
 
+  // Write barrier support for address[start : start + len[ = o.
+  static inline void RecordWrites(Address address, int start, int len);
+
   // Given an address occupied by a live code object, return that object.
   static Object* FindCodeObject(Address a);