Do not coerce lastIndex of a global RegExp in @@match and @@replace.

test/mjsunit/es6/regexp-replace-lastindex.js

Index: test/mjsunit/es6/regexp-replace-lastindex.js
diff --git a/test/mjsunit/es6/regexp-replace-lastindex.js b/test/mjsunit/es6/regexp-replace-lastindex.js
new file mode 100644
index 0000000000000000000000000000000000000000..08217e63896bbd43b3a10945db6d5d1bd6a4df44
--- /dev/null
+++ b/test/mjsunit/es6/regexp-replace-lastindex.js
@@ -0,0 +1,11 @@
+// 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.
+
+// Tests that lastIndex of a global RegExp is overwritten as per
+// ECMA-262 6.0 21.2.5.8 step 10.c.
+
+var global = /./g;
+global.lastIndex = { valueOf: function() { assertUnreachable(); } };
+assertEquals("X", "x".replace(global, function(a) { return "X"; }));
+assertEquals(0, global.lastIndex);