ArrayConcat regression recover after r20312 (appeared on dromaeo benchmarks).

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index 4a393cb27c50e057bf74f69efb99e046dfb1cb45..b5162fd1ab8159e4ac2e4d0612a029a62950bce3 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -987,12 +987,12 @@ BUILTIN(ArrayConcat) {
   Handle<FixedArrayBase> storage(result_array->elements(), isolate);
   ElementsAccessor* accessor = ElementsAccessor::ForKind(elements_kind);
   for (int i = 0; i < n_arguments; i++) {
-    // TODO(ishell): It is crucial to keep |array| as a raw pointer to avoid
-    // performance degradation. Revisit this later.
+    // It is crucial to keep |array| in a raw pointer form to avoid performance
+    // degradation.
     JSArray* array = JSArray::cast(args[i]);
     int len = Smi::cast(array->length())->value();
-    ElementsKind from_kind = array->GetElementsKind();
     if (len > 0) {
+      ElementsKind from_kind = array->GetElementsKind();
       accessor->CopyElements(array, 0, from_kind, storage, j, len);
       j += len;
     }