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

runtime/bin/file_patch.dart

Index: runtime/bin/file_patch.dart
diff --git a/runtime/bin/file_patch.dart b/runtime/bin/file_patch.dart
index e41f2f5efe857379bb6810476e12d1895d6c226e..ed8e3a0590f1807b849b4536782932d8df5c3a4e 100644
--- a/runtime/bin/file_patch.dart
+++ b/runtime/bin/file_patch.dart
@@ -124,8 +124,8 @@ patch class _FileSystemWatcher {
       assert(_watcherPath.count > 0);
       _watcherPath.count--;
       if (_watcherPath.count == 0) {
-        _pathWatchedEnd();
         _unwatchPath(_id, _watcherPath.pathId);
+        _pathWatchedEnd();
         _idMap.remove(_watcherPath.pathId);
       }
       _watcherPath = null;