Add debugging prints to the Mac OS directory watcher.

This is a temporary CL that will make it easier to track down and fix the errors
in the mac watcher.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//107403002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30919 260f80e4-7a28-3924-810f-c04153c831b5