Revert watcher changes for mac.

We ended up with a 1.6 dependency, which I don't know if is a good idea or not.

Revert "Mac no longer fire bogus events. Fix Mac watcher."
Revert "Don't use kFSEventStreamCreateFlagNoDefer as it messes with event flushing."

BUG=

Review URL: https://codereview.chromium.org//325963005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@37173 260f80e4-7a28-3924-810f-c04153c831b5