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

sdk/lib/core/sort.dart

 // Copyright (c) 2011, 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.
 
-// TODO(ajohnsen): Remove once coreimpl is eliminated.
+// TODO(ajohnsen): Remove once dart2js intercepters can work without it.
 /**
  * WARNING: This method is temporary and will go away soon.
  */
 void coreSort(List l, int compare(a, b)) => _Sort.sort(l, compare);
 
 /**
  * Dual-Pivot Quicksort algorithm.
  *
  * This class implements the dual-pivot quicksort algorithm as presented in
  * Vladimir Yaroslavskiy's paper.
@@ skipping 323 matching lines @@
     } else {
       // The second partition looks as follows:
       // [  *  |  >= pivot1 && <= pivot2  | * ]
       //        ^                        ^
       //       less                    great
       // Simply sort it by recursive descent.
       _doSort(a, less, great, compare);
     }
   }
 }