Let the stream of File:openRead handle cancel correctly.

tests/standalone/io/directory_list_pause_test.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.
 
 import "dart:async";
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
@@ skipping 64 matching lines @@
 
 void testListIsEmpty() {
   asyncStart();
   // TOTAL should be bigger the our directory listing buffer.
   const int TOTAL = 128;
   Directory.systemTemp.createTemp('dart_directory_list_pause').then((d) {
     for (int i = 0; i < TOTAL; i++) {
       new Directory("${d.path}/$i").createSync();
       new File("${d.path}/$i/file").createSync();
     }
+    // isEmpty will cancel the stream after first data event.
     d.list(recursive: true).isEmpty.then((empty) {
       Expect.isFalse(empty);
       d.deleteSync(recursive: true);
       asyncEnd();
     });
   });
 }
 
 void main() {
   testPauseList();
   testPauseResumeCancelList();
   testListIsEmpty();
 }