Roll forward r30205.

Unlike previous versions of this CL, the debug info will only be printed when
running directory_watcher/mac_os_test, so it won't break the pub bots.

R=rnystrom@google.com
BUG=15042

Review URL: https://codereview.chromium.org//71353011

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30254 260f80e4-7a28-3924-810f-c04153c831b5