Be sure we don't forward an error to a closed event stream.

pkg/watcher/lib/src/directory_watcher/polling.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.
 
 library watcher.directory_watcher.polling;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:crypto/crypto.dart';
@@ skipping 54 matching lines @@
   AsyncQueue<String> _filesToProcess;
 
   /// The set of files that have been seen in the current directory listing.
   ///
   /// Used to tell which files have been removed: files that are in [_statuses]
   /// but not in here when a poll completes have been removed.
   final _polledFiles = new Set<String>();
 
   _PollingDirectoryWatcher(this.directory, this._pollingDelay) {
     _filesToProcess = new AsyncQueue<String>(_processFile,
-        onError: _events.addError);
+        onError: (e, stackTrace) {
+      if (!_events.isClosed) _events.addError(e, stackTrace);
+    });
 
     _poll();
   }
 
   void close() {
     _events.close();
 
     // If we're in the middle of listing the directory, stop.
     if (_listSubscription != null) _listSubscription.cancel();
 
@@ skipping 121 matching lines @@
 class _FileStatus {
   /// The last time the file was modified.
   DateTime modified;
 
   /// The SHA-1 hash of the contents of the file.
   List<int> hash;
 
   _FileStatus(this.modified, this.hash);
 }