Move CopyElements to the accessor of the target.

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index d9f8d15411ad91be9dd141cfe9d4534eeb9782ea..18325588c9a7d54b3414f376e80fb4b8083935cd 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -1220,13 +1220,14 @@ BUILTIN(ArrayConcat) {
 
   int j = 0;
   FixedArrayBase* storage = result_array->elements();
+  ElementsAccessor* accessor = ElementsAccessor::ForKind(elements_kind);
   for (int i = 0; i < n_arguments; i++) {
     JSArray* array = JSArray::cast(args[i]);
     int len = Smi::cast(array->length())->value();
+    ElementsKind from_kind = array->GetElementsKind();
     if (len > 0) {
-      ElementsAccessor* accessor = array->GetElementsAccessor();
       MaybeObject* maybe_failure =
-          accessor->CopyElements(array, 0, storage, elements_kind, j, len);
+          accessor->CopyElements(array, 0, storage, from_kind, j, len);
       if (maybe_failure->IsFailure()) return maybe_failure;
       j += len;
     }