The ArrayConcat builtin didn't respect @@isConcatSp

src/elements.cc

Index: src/elements.cc
diff --git a/src/elements.cc b/src/elements.cc
index 85eb3f266ecdb135e0ac8081d2b0d08bdd330477..917a31d0b5c85f3a1d493f820ccea2e0b6c3780f 100644
--- a/src/elements.cc
+++ b/src/elements.cc
@@ -1946,5 +1946,23 @@ void ElementsAccessor::TearDown() {
 
 
 ElementsAccessor** ElementsAccessor::elements_accessors_ = NULL;
+
+
+bool IsConcatSpreadable(Isolate* isolate, Handle<Object> obj) {
+  HandleScope handle_scope(isolate);
+  if (!obj->IsSpecObject()) return false;
+  if (FLAG_harmony_concat_spreadable) {
+    Handle<Symbol> key(isolate->factory()->is_concat_spreadable_symbol());
+    Handle<Object> value;
+    MaybeHandle<Object> maybeValue =
+        i::Runtime::GetObjectProperty(isolate, obj, key);
+    if (maybeValue.ToHandle(&value)) {
+      if (!value->IsUndefined()) {
+        return value->BooleanValue();
+      }
+    }
+  }
+  return obj->IsJSArray();
+}
 }  // namespace internal
 }  // namespace v8