Move js library back into compiler directory.

sdk/lib/_internal/compiler/js_lib/regexp_helper.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.
 
 part of _js_helper;
 
 // Helper method used by internal libraries.
 regExpGetNative(JSSyntaxRegExp regexp) => regexp._nativeRegExp;
 
 /**
@@ skipping 198 matching lines @@
     _current = null;
     _string = null;  // Marks iteration as ended.
     return false;
   }
 }
 
 /** Find the first match of [regExp] in [string] at or after [start]. */
 Match firstMatchAfter(JSSyntaxRegExp regExp, String string, int start) {
   return regExp._execGlobal(string, start);
 }