Fix issue where we would double unwatch the path.

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 125 matching lines @@
       } else {
         assert(false);
       }
       if (stop) socket.close();
       return events;
     })
     .listen(_controller.add, onDone: _cancel);
   }
 
   void _cancel() {
-    _unwatchPath();
     if (_subscription != null) {
+      _unwatchPath();
       _subscription.cancel();
+      _subscription = null;
     }
     _controller.close();
   }
 
   Stream<FileSystemEvent> get stream => _controller.stream;
 
   static bool get isSupported native "FileSystemWatcher_IsSupported";
 
   int _watchPath(String path, int events, bool recursive)
       native "FileSystemWatcher_WatchPath";
   void _unwatchPath() native "FileSystemWatcher_UnwatchPath";
   List _readEvents() native "FileSystemWatcher_ReadEvents";
 }
 
 Uint8List _makeUint8ListView(Uint8List source, int offsetInBytes, int length) {
   return new Uint8List.view(source.buffer, offsetInBytes, length);
 }