Fix a race condition in file watcher.

lib/src/file_watcher/native.dart

 // Copyright (c) 2015, 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.file_watcher.native;
 
 import 'dart:async';
 import 'dart:io';
 
 import '../file_watcher.dart';
@@ skipping 42 matching lines @@
     if (batch.any((event) => event.type == FileSystemEvent.DELETE)) {
       // If the file is deleted, the underlying stream will close. We handle
       // emitting our own REMOVE event in [_onDone].
       return;
     }
 
     _eventsController.add(new WatchEvent(ChangeType.MODIFY, path));
   }
 
   _onDone() async {
-    // If the file exists now, it was probably removed and quickly replaced;
-    // this can happen for example when another file is moved on top of it.
-    // Re-subscribe and report a modify event.
-    if (await new File(path).exists()) {
+    var fileExists = await new File(path).exists();
+
+    // Check for this after checking whether the file exists because it's
+    // possible that [close] was called between [File.exists] being called and
+    // it completing.
+    if (_eventsController.isClosed) return;
+
+    if (fileExists) {
+      // If the file exists now, it was probably removed and quickly replaced;
+      // this can happen for example when another file is moved on top of it.
+      // Re-subscribe and report a modify event.
       _eventsController.add(new WatchEvent(ChangeType.MODIFY, path));
       _listen();
     } else {
       _eventsController.add(new WatchEvent(ChangeType.REMOVE, path));
       close();
     }
   }
 
   void close() {
     if (_subscription != null) _subscription.cancel();
     _subscription = null;
     _eventsController.close();
   }
 }