Add check to watcher, to avoid null-error if unknown path is removed.

BUG=
R=kevmoo@google.com

Review URL: https://codereview.chromium.org//428493004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@38846 260f80e4-7a28-3924-810f-c04153c831b5