Regression test for JSRegExp literals sharing.

test/mjsunit/regress/regress-crbug-548580.js

+// 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-regexps
+
+function store(v) {
+  var re = /(?=[d#.])/;
+  re.a = v;
+  return re;
+}
+
+var re1 = store(undefined);
+var re2 = store(42);
+
+assertEquals(re1.a, undefined);
+assertEquals(re2.a, 42);