Avoid race in _FileSystemWatcher if subscription closed while event queued.

runtime/bin/file_patch.dart

 // Copyright (c) 2012, 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.
 
 patch class _File {
   /* patch */ static _exists(String path) native "File_Exists";
   /* patch */ static _create(String path) native "File_Create";
   /* patch */ static _createLink(String path, String target)
       native "File_CreateLink";
   /* patch */ static _linkTarget(String path) native "File_LinkTarget";
@@ skipping 175 matching lines @@
             add(event[4], new FileSystemDeleteEvent._(getPath(event), isDir));
           }
         }
         int eventCount;
         do {
           eventCount = 0;
           for (var event in _readEvents(id, pathId)) {
             if (event == null) continue;
             eventCount++;
             int pathId = event[4];
+            if (!_idMap.containsKey(pathId)) {

[Søren Gjesse, 2015/03/13 12:18:00]

I am not sure this is 100% safe, but I have not scrutinized the code.

The pathId is something returned from an OS call (on Linux that is returned from
inotify_add_watch). This id can be reused, so if unwatching something and then
watching something new the OS can reuse the id.

Can this result in events for the first registration being reported on the new
one?

+              // Path is no longer being wathed.
+              continue;
+            }
             bool isDir = getIsDir(event);
             var path = getPath(event);
             if ((event[0] & FileSystemEvent.CREATE) != 0) {
               add(event[4], new FileSystemCreateEvent._(path, isDir));
             }
             if ((event[0] & FileSystemEvent.MODIFY) != 0) {
               add(event[4], new FileSystemModifyEvent._(path, isDir, true));
             }
             if ((event[0] & FileSystemEvent._MODIFY_ATTRIBUTES) != 0) {
               add(event[4], new FileSystemModifyEvent._(path, isDir, false));
@@ skipping 155 matching lines @@
   void _pathWatchedEnd() {
     _subscription.cancel();
     _controller.close();
   }
 }
 
 
 Uint8List _makeUint8ListView(Uint8List source, int offsetInBytes, int length) {
   return new Uint8List.view(source.buffer, offsetInBytes, length);
 }