Fix copy-on-write assert by setting the new array map early.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index cfb3b6a0300d10f9da474a869ab9e7edef82c553..8a11530d8ce086ed579a0189656f680e9b4d4071 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -498,7 +498,12 @@ class Heap : public AllStatic {
 
   // Make a copy of src and return it. Returns
   // Failure::RetryAfterGC(requested_bytes, space) if the allocation failed.
-  MUST_USE_RESULT static Object* CopyFixedArray(FixedArray* src);
+  MUST_USE_RESULT static inline Object* CopyFixedArray(FixedArray* src);
+
+  // Make a copy of src, set the map, and return the copy. Returns
+  // Failure::RetryAfterGC(requested_bytes, space) if the allocation failed.
+  MUST_USE_RESULT static Object* CopyFixedArrayWithMap(FixedArray* src,
+                                                       Map* map);
 
   // Allocates a fixed array initialized with the hole values.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation