Use 'Directory.watch' on Windows in pkg/watcher, instead of pooling.

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 106 matching lines @@
     _watcherPath = _idMap[pathId];
     _watcherPath.count++;
     _pathWatched().pipe(_broadcastController);
   }
 
   void _cancel() {
     if (_watcherPath != null) {
       assert(_watcherPath.count > 0);
       _watcherPath.count--;
       if (_watcherPath.count == 0) {
+        _unwatchPath(_id, _watcherPath.pathId);
         _pathWatchedEnd();
-        _unwatchPath(_id, _watcherPath.pathId);
         _idMap.remove(_watcherPath.pathId);
       }
       _watcherPath = null;
     }
     if (_idMap.isEmpty && _id != null) {
       _closeWatcher(_id);
       _doneWatcher();
       _id = null;
     }
   }
@@ skipping 220 matching lines @@
   void _pathWatchedEnd() {
     _subscription.cancel();
     _controller.close();
   }
 }
 
 
 Uint8List _makeUint8ListView(Uint8List source, int offsetInBytes, int length) {
   return new Uint8List.view(source.buffer, offsetInBytes, length);
 }