Wrap Directory.watch on Mac OS for the watcher package.

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 3070d790636ec19323e1cad670245021234e06bc..e18c7e8131919db141932865c07618a89c924e92 100644
--- a/pkg/watcher/test/utils.dart
+++ b/pkg/watcher/test/utils.dart
@@ -137,6 +137,8 @@ void startWatcher({String dir}) {
       // people think it might be the root cause.
       if (currentSchedule.errors.isEmpty) {
         expect(allEvents, hasLength(numEvents));
+      } else {
+        currentSchedule.addDebugInfo("Events fired:\n${allEvents.join('\n')}");
       }
     }, "reset watcher");