label must propagate to here.
take a peak at: https://github.com/tildeio/rsvp.js/commit/9875d9f76ecbeb320c1f4d8aedbd23a3e0af4e64 and 

- https://github.com/tildeio/rsvp.js/blob/master/lib/rsvp/hash.js#L13
- https://github.com/tildeio/rsvp.js/blob/master/lib/rsvp/hash.js#L45