Use symbols instead of hidden properties for i18n markers.

test/mjsunit/test-hidden-string.js

Index: test/mjsunit/test-hidden-string.js
diff --git a/test/mjsunit/test-hidden-string.js b/test/mjsunit/test-hidden-string.js
deleted file mode 100644
index a5d32c839ea38e08abedc2d95542009accdec36c..0000000000000000000000000000000000000000
--- a/test/mjsunit/test-hidden-string.js
+++ /dev/null
@@ -1,11 +0,0 @@
-// Copyright 2014 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.
-
-// Flags: --allow-natives-syntax
-
-var o = {};
-%SetHiddenProperty(o, "test", 1);
-// Create non-internalized ""
-var empty = "a".substring(1, 1);
-assertEquals(undefined, o[empty]);