This is introspection, which can be provided elsewhere or by specific
implementations. Together with the documented requirement (in Eventable#on)
that listeners must be notified of automatic un-hooking, and that systems
handling listeners will always be aware of when listeners are
hooked/unhooked, this method should not be necessary.
Including this method in Eventable also introduces incompatibility with
Node.js' Event API, which is undesirable.
As a conscious TODO: need to properly handle attaching the same event
listener multiple times (define its behavior), and maybe even provide an API
to configure that behavior in some way.
It will take some getting used to (and it's a little sad to see some of the
long-standing conventions of ECMAScript vanish before my eyes), but it's
great thus far.
Mike Gerwitz