Add a bunch of debugging prints to the Mac OS watcher.
Add a bunch of debugging prints to the Mac OS watcher.

This should help debug the intermittent, difficult-to-reproduce issues
on the vm-mac-release bot.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//68293005

git-svn-id: c53bd6455474c0a826d309663cf600546f7b0963@30181 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/watcher