Add a missing import to pkg/watcher.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//104033003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30914 260f80e4-7a28-3924-810f-c04153c831b5

pkgs/watcher/lib/src/directory_watcher/mac_os.dart

diff --git a/pkgs/watcher/lib/src/directory_watcher/mac_os.dart b/pkgs/watcher/lib/src/directory_watcher/mac_os.dart
index 16de1d9..c3e03b8 100644
--- a/pkgs/watcher/lib/src/directory_watcher/mac_os.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/mac_os.dart
@@ -7,6 +7,8 @@
 import 'dart:async';
 import 'dart:io';
 
+import 'package:stack_trace/stack_trace.dart';
+
 import '../constructable_file_system_event.dart';
 import '../path_set.dart';
 import '../utils.dart';