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

pkg/watcher/test/directory_watcher/windows_test.dart

+// Copyright (c) 2014, 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.
+
+import 'package:scheduled_test/scheduled_test.dart';
+import 'package:watcher/src/directory_watcher/windows.dart';
+import 'package:watcher/watcher.dart';
+
+import 'shared.dart';
+import '../utils.dart';
+
+main() {
+  initConfig();
+
+  watcherFactory = (dir) => new WindowsDirectoryWatcher(dir);
+
+  setUp(createSandbox);
+
+  sharedTests();
+
+  test('DirectoryWatcher creates a WindowsDirectoryWatcher on Windows', () {
+    expect(new DirectoryWatcher('.'),
+        new isInstanceOf<WindowsDirectoryWatcher>());
+  });
+}
+