Cancelable promises proposal

[C. Scott Ananian]

IMO promises should reject if they are "cancelled".  The same method that
gave you the Promise (for example, an AJAX API) can give you a capability
to "cancel" the Promise, ie cancel the pending network operation and reject
the Promise (rejecting is a no-op if the operation races with cancellation
and manages to resolve first, naturally).

I'm not opposed to setting up a defacto API for the cancellation
capability, but I don't think this needs to be part of EcmaScript.  If you
want to define a `CancelablePromise` subclass which has a
`CancelablePromise#cancel()` method, feel free.  But I don't think it
should be tightly tied to the Promise spec at all.  The capability to
cancel an operation should be first-class, and able to be passed around
independent of the Promise which gives the future result of the operation.

I don't think you need to define an "unlisten" API, either.  Just discard
the resolve/reject capabilities for the Promise in question.  The last time
I audited the spec, I convinced myself that this would ensure that all
chained promises were also garbage and would be safely collected.  (The
only pointers to chained promises are held by the resolve/reject
capabilities.)
  --scott
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