Filter old to new references when moving parts of an object.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 9281fa9d72e354bfc4e16d035bcc14e71eb7a158..58c6c5d6f5ef26e36aea5b3677ae50fcdcc001b1 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -805,6 +805,10 @@ class Heap {
   // Please note this does not perform a garbage collection.
   MUST_USE_RESULT MaybeObject* AllocateUninitializedFixedArray(int length);
 
+  // Move len elements within a given array from src_index index to dst_index
+  // index.
+  void MoveElements(FixedArray* array, int dst_index, int src_index, int len);
+
   // Make a copy of src and return it. Returns
   // Failure::RetryAfterGC(requested_bytes, space) if the allocation failed.
   MUST_USE_RESULT inline MaybeObject* CopyFixedArray(FixedArray* src);