Switch from interfaces to abstract classes in dart:io.

runtime/bin/path_impl.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 _Path implements Path {
   final String _path;
 
-  const _Path(String source) : _path = source;
+  _Path(String source) : _path = source;
   _Path.fromNative(String source) : _path = _clean(source);
 
   int hashCode() => _path.hashCode();
 
   static String _clean(String source) {
     switch (Platform.operatingSystem) {
       case 'windows':
         return _cleanWindows(source);
       default:
         return source;
@@ skipping 191 matching lines @@
     if (pos < 0) return new Path('');
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     return new Path((pos > 0) ? _path.substring(0, pos) : '/');
   }
 
   String get filename {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }