Guard the property RegExp.prototype.unicode behind --harmony-regexp-unicode

test/mjsunit/es6/no-unicode-regexp-flag.js

Index: test/mjsunit/es6/no-unicode-regexp-flag.js
diff --git a/test/mjsunit/es6/no-unicode-regexp-flag.js b/test/mjsunit/es6/no-unicode-regexp-flag.js
new file mode 100644
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+++ b/test/mjsunit/es6/no-unicode-regexp-flag.js
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+// 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.
+
+// Before Unicode RegExps are shipped, we shouldn't have the 'unicode'
+// property on RegExp.prototype, or read it from 'flags'.
+// mjsunit/es6/regexp-flags tests that the property is there when the
+// flag is on.
+
+// Flags: --harmony-regexp
+
+'use strict';
+
+assertFalse(RegExp.prototype.hasOwnProperty('unicode'));
+
+// If we were going to be really strict, we could have a test like this,
+// with the assertTrue replaced by assertFalse, since flags shouldn't
+// Get the 'unicode' property. However, it is probably OK to omit this
+// detailed fix.
+var x = /a/;
+var y = false;
+Object.defineProperty(x, 'unicode', { get() { y = true; } });
+assertEquals("", x.flags);
+assertTrue(y);