builtins.cc return PackedElementsKind where applicable

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index a68a15ad5d45cefaf2662ac829bec44fa21834df..dacd7ae5dbf0237a9b9900e42a2a2c81dafbaa70 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -526,8 +526,8 @@ BUILTIN(ArraySlice) {
       (relative_end < 0) ? Max(len + relative_end, 0) : Min(relative_end, len);
 
   if (actual_end <= actual_start) {
-    Handle<JSArray> result_array =
-        isolate->factory()->NewJSArray(object->GetElementsKind(), 0, 0);
+    Handle<JSArray> result_array = isolate->factory()->NewJSArray(
+        GetPackedElementsKind(object->GetElementsKind()), 0, 0);
     return *result_array;
   }