Add fromUri constructor to File, Directory and Link

sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, 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 dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 215 matching lines @@
   final String path;
 
   // Constructor for file.
   _File(String this.path) {
     if (path is! String) {
       throw new ArgumentError('${Error.safeToString(path)} '
                               'is not a String');
     }
   }
 
+  // Constructor for file from a URI.
+  factory _File.fromUri(Uri uri) => new _File(uri.toFilePath());
+
   Future<bool> exists() {
     return _IOService.dispatch(_FILE_EXISTS, [path]).then((response) {
       if (_isErrorResponse(response)) {
         throw _exceptionFromResponse(response, "Cannot check existence", path);
       }
       return response;
     });
   }
 
   external static _exists(String path);
@@ skipping 633 matching lines @@
 
   void _checkAvailable() {
     if (_asyncDispatched) {
       throw new FileSystemException("An async operation is currently pending", path);
     }
     if (closed) {
       throw new FileSystemException("File closed", path);
     }
   }
 }