[regexp] Implement regexp groups as wrapper.

test/mjsunit/regress/regress-5845.js

Index: test/mjsunit/regress/regress-5845.js
diff --git a/test/mjsunit/regress/regress-5845.js b/test/mjsunit/regress/regress-5845.js
new file mode 100644
index 0000000000000000000000000000000000000000..45a580b24ff52b87f0aab369e2eed0c88fec62a7
--- /dev/null
+++ b/test/mjsunit/regress/regress-5845.js
@@ -0,0 +1,6 @@
+// Copyright 2017 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.
+
+assertDoesNotThrow('/(?:(?=(foo)))?/u.exec("foo")');
+assertThrows('/(?=(foo))?/u.exec("foo")');