Move JSSyntaxRegExp to core as a private member. This removes the last refrences to dart:coreimpl.

sdk/lib/coreimpl/regexp.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-class JSSyntaxRegExp implements RegExp {
-  external JSSyntaxRegExp(String pattern,
-                          {bool multiLine: false,
-                           bool ignoreCase: false});
-  external Match firstMatch(String str);
-  external Iterable<Match> allMatches(String str);
-  external bool hasMatch(String str);
-  external String stringMatch(String str);
-  external String get pattern;
-  external bool get multiLine;
-  external bool get ignoreCase;
-}