Guard @@isConcatSpreadable behind a flag

src/runtime/runtime-array.cc

Index: src/runtime/runtime-array.cc
diff --git a/src/runtime/runtime-array.cc b/src/runtime/runtime-array.cc
index b2865dcbdf323b6829fec9e02d2f262036fcd0c1..d00df71576bf27edcee2131a8c0cd5656e314d8c 100644
--- a/src/runtime/runtime-array.cc
+++ b/src/runtime/runtime-array.cc
@@ -729,13 +729,15 @@ static bool IterateElements(Isolate* isolate, Handle<JSObject> receiver,
 static bool IsConcatSpreadable(Isolate* isolate, Handle<Object> obj) {
   HandleScope handle_scope(isolate);
   if (!obj->IsSpecObject()) return false;
-  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();
+  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();