Correctly pass errors to the stream, when watching file system entities.

tests/standalone/io/file_system_watcher_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 238 matching lines @@
   file.createSync();
 
   new Timer(const Duration(milliseconds: 300), () {
     sub.cancel();
     asyncEnd();
     dir.deleteSync(recursive: true);
   });
 }
 
 
+void testWatchNonExisting() {
+  asyncStart();
+  new Directory('__some_none_existing_dir__').watch()
+    .listen((_) {
+      Expect.fail('unexpected error');
+    }, onError: (e) {
+      asyncEnd();
+      Expect.isTrue(e is FileSystemException);
+    });
+}
+
+
 void main() {
   if (!FileSystemEntity.isWatchSupported) return;
   testWatchCreateFile();
   testWatchModifyFile();
   testWatchMoveFile();
   testWatchDeleteFile();
   testWatchDeleteDir();
   testWatchOnlyModifyFile();
   testMultipleEvents();
   testWatchNonRecursive();
+  testWatchNonExisting();
 }