Guard @@isConcatSpreadable behind a flag

src/harmony-concat-spreadable.js

Index: src/harmony-concat-spreadable.js
diff --git a/src/harmony-concat-spreadable.js b/src/harmony-concat-spreadable.js
new file mode 100644
index 0000000000000000000000000000000000000000..362701c12304d8f66d7b9dc1515369e0bf868fc5
--- /dev/null
+++ b/src/harmony-concat-spreadable.js
@@ -0,0 +1,16 @@
+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(function(global, utils) {
+
+'use strict';
+
+%CheckIsBootstrapping();
+
+utils.InstallConstants(global.Symbol, [
+  // TODO(littledan): Move to symbol.js when shipping
+  "isConcatSpreadable", symbolIsConcatSpreadable
+]);
+
+})