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

pkg/watcher/lib/src/directory_watcher.dart

 // Copyright (c) 2013, 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.directory_watcher;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'watch_event.dart';
 import 'directory_watcher/linux.dart';
 import 'directory_watcher/mac_os.dart';
+import 'directory_watcher/windows.dart';
 import 'directory_watcher/polling.dart';
 
 /// Watches the contents of a directory and emits [WatchEvent]s when something
 /// in the directory has changed.
 abstract class DirectoryWatcher {
   /// The directory whose contents are being monitored.
   String get directory;
 
   /// The broadcast [Stream] of events that have occurred to files in
   /// [directory].
@@ skipping 26 matching lines @@
   /// use it. Otherwise, it will fall back to a [PollingDirectoryWatcher].
   ///
   /// If [_pollingDelay] is passed, it specifies the amount of time the watcher
   /// will pause between successive polls of the directory contents. Making this
   /// shorter will give more immediate feedback at the expense of doing more IO
   /// and higher CPU usage. Defaults to one second. Ignored for non-polling
   /// watchers.
   factory DirectoryWatcher(String directory, {Duration pollingDelay}) {
     if (Platform.isLinux) return new LinuxDirectoryWatcher(directory);
     if (Platform.isMacOS) return new MacOSDirectoryWatcher(directory);
+    if (Platform.isWindows) return new WindowsDirectoryWatcher(directory);
     return new PollingDirectoryWatcher(directory, pollingDelay: pollingDelay);
   }
 }