Make RegExp use ToLength on lastIndex when flag is turned on

test/mjsunit/harmony/regexp-tolength.js

Index: test/mjsunit/harmony/regexp-tolength.js
diff --git a/test/mjsunit/harmony/regexp-tolength.js b/test/mjsunit/harmony/regexp-tolength.js
new file mode 100644
index 0000000000000000000000000000000000000000..d9e967ba27dcbb3a1ee0f9a096b8cf6870c21d63
--- /dev/null
+++ b/test/mjsunit/harmony/regexp-tolength.js
@@ -0,0 +1,19 @@
+// 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.
+
+// Flags: --harmony-tolength
+
+'use strict';
+
+let regexp = /x/g;
+
+regexp.lastIndex = -1;
+
+assertTrue(regexp.test("axb"));
+assertEquals(2, regexp.lastIndex);
+
+regexp.lastIndex = -1;
+
+assertEquals("x", regexp.exec("axb")[0]);
+assertEquals(2, regexp.lastIndex);