Let the stream of File:openRead handle cancel correctly.

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 36 matching lines @@
                                      cancelOnError: cancelOnError);
   }
 
   void _setupController() {
     _controller = new StreamController<List<int>>(sync: true,
         onListen: _start,
         onPause: () => _paused = true,
         onResume: _resume,
         onCancel: () {
           _unsubscribed = true;
-          _closeFile();
+          return _closeFile();
         });
   }
 
   Future _closeFile() {
     if (_readInProgress) {
       return _closeCompleter.future;
     }
     if (_openedFile != null) {
       _openedFile.close()
           .then(_closeCompleter.complete,
@@ skipping 799 matching lines @@
 
   void _checkAvailable() {
     if (_asyncDispatched) {
       throw new FileSystemException("An async operation is currently pending", path);
     }
     if (closed) {
       throw new FileSystemException("File closed", path);
     }
   }
 }