Fix error handling in watcher.

pkg/watcher/lib/src/directory_watcher.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;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:crypto/crypto.dart';
@@ skipping 116 matching lines @@
       _filesToProcess.add(null);
     }
 
     var stream = new Directory(directory).list(recursive: true);
     _listSubscription = stream.listen((entity) {
       assert(_state != _WatchState.UNSUBSCRIBED);
 
       if (entity is! File) return;
       _filesToProcess.add(entity.path);
     }, onError: (error) {
-      if (isDirectoryNotFoundException(error)) {
-        // If the directory doesn't exist, we end the listing normally, which
-        // has the desired effect of marking all files that were in the
-        // directory as being removed.
-        endListing();
-        return;
+      if (!isDirectoryNotFoundException(error)) {
+        // It's some unknown error. Pipe it over to the event stream so the
+        // user can see it.
+        _events.addError(error);
       }
 
-      // It's some unknown error. Pipe it over to the event stream so we don't
-      // take down the whole isolate.
-      _events.addError(error);
-    }, onDone: endListing);
+      // When an error occurs, we end the listing normally, which has the
+      // desired effect of marking all files that were in the directory as
+      // being removed.
+      endListing();
+    }, onDone: endListing, cancelOnError: true);
   }
 
   /// Processes [file] to determine if it has been modified since the last
   /// time it was scanned.
   Future _processFile(String file) {
     assert(_state != _WatchState.UNSUBSCRIBED);
 
     // `null` is the sentinel which means the directory listing is complete.
     if (file == null) return _completePoll();
 
@@ skipping 114 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);
 }