Split dart:builtin into dart:builtin and dart:io.

frog/file_system.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
 /** Abstraction for file systems and utility functions to manipulate paths. */
 #library('file_system');
 
 /**
  * Abstraction around file system access to work in a variety of different
  * environments.
  */
@@ skipping 52 matching lines @@
 String basename(String path) {
   path = canonicalizePath(path);
 
   int lastSlash = path.lastIndexOf('/', path.length);
   if (lastSlash == -1) {
     return path;
   } else {
     return path.substring(lastSlash + 1);
   }
 }