Move core and coreimpl from corelib/ to lib/core and lib/coreimpl.

lib/coreimpl/string.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.
 
 class StringImplementation {
   /**
    * Factory implementation of String.fromCharCodes:
    * Allocates a new String for the specified [charCodes].
    */
   factory String.fromCharCodes(List<int> charCodes) {
     return _fromCharCodes(charCodes);
   }
 
   external static String _fromCharCodes(List<int> charCodes);
 
   /**
    * Joins all the given strings to create a new string.
    */
   external static String join(List<String> strings, String separator);
 
   /**
    * Concatenates all the given strings to create a new string.
    */
   external static String concatAll(List<String> strings);
 
 }