Fix missing events for files with directory prefix (dart-lang/watcher#84)

Fixes dart-lang/watcher#83

If any event had the same leading characters as a directory that was
also changed it would get filtered out. For example if both `lib/b/` and
`lib/b.dart` have change events we'd lose the events for `lib/b.dart`
because it shared a prefix with a directory, even though it isn't
contained within that directory.