Fix an analyzer error in the linux directory watcher.
Fix an analyzer error in the linux directory watcher.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//65533005

git-svn-id: c53bd6455474c0a826d309663cf600546f7b0963@30093 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/watcher