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

pkg/watcher/test/utils.dart

Index: pkg/watcher/test/utils.dart
diff --git a/pkg/watcher/test/utils.dart b/pkg/watcher/test/utils.dart
index b520bbd9a94ee75a08816a6d54e5622a6d1ff862..8b660e845822a464a37340e9529ca3a37a022466 100644
--- a/pkg/watcher/test/utils.dart
+++ b/pkg/watcher/test/utils.dart
@@ -71,6 +71,11 @@ void createSandbox() {
   // Delete the sandbox when done.
   currentSchedule.onComplete.schedule(() {
     if (_sandboxDir != null) {
+      // TODO(rnystrom): Issue 19155. The watcher should already be closed when
+      // we clean up the sandbox.
+      if (_watcherEvents != null) {
+        _watcherEvents.close();
+      }
       new Directory(_sandboxDir).deleteSync(recursive: true);
       _sandboxDir = null;
     }