Fix registerCustomWatcher to allow nulls (dart-lang/watcher#107)

The idea behind the closures passed to `registerCustomWatcher` is that
they should return `null` if the particular implementation does not
support the provided path (in which case we should fallback to other
implementations).

Modified a test to check this.
2 files changed
tree: dd927e0169be717fd58c9fe436d3bbd4f189ac0f
  1. pkgs/