Fix memory leak

[ForbesLindesay]

This is a crude attempt at fixing the memory leak caused by code like:

function next() {
  return new Promise(function (resolve) {
   setImmediate(resolve)
  }).then(next)
}
next()

In theory this should be a viable way of running a continuous polling operation.

[ForbesLindesay]

P.S. don't merge this yet. We should make "handle" some kind of private symbol if we are going to take this approach.

[stefanpenner]

just for note, there is an observable difference that may or may not be tolerable.

promises-aplus/promises-spec#179 (comment)

[ForbesLindesay]

That's interesting. I think that probably is tolerable, but I do wonder if we could fix that somehow.

[edef1c]

@ForbesLindesay I've been contemplating doing this for a long time, and I probably have a local branch around doing a similar thing. Collapsing promise chains like this should be a nice perf boost.
My own plan was to stick a few private symbols on the then function, containing the state. Handlers could be offloaded onto that promise straight away, and synchronous inspection (from the then being invoked, instead of dispatching) becomes feasible too. I can't find my version of the code right now, but I have a feeling it'll be faster than invoking further dynamic dispatch.

[edef1c]

I think depending on es6-symbol and sticking everything under then should resolve privacy/encapsulation concerns. I'll have to run some perf tests on the cost of messing with function properties. I'm not sure how those'll turn out, but I'd rather not lose the semantic niceties of that to the Perf Monster.

[ForbesLindesay]

The encapsulation is just as good if we use es6-symbol on this rather than this.then so I would suggest sticking with that as it seems simpler.

[edef1c]

Sure, but .then can freely be moved around and replaced. We could get some very weird behaviour on subclasses. .then is where the behaviour of .then is defined, basically.

[ForbesLindesay]

I now believe this is ready to merge, and it fixes a major memory leak. @nathan7 if you can review this asap and do the honours, that would be great.

[ForbesLindesay]

I'm merging this now to get unblocked, but I'd still love a review if someone gets a chance.

[edef1c]

Looks good to me, @ForbesLindesay! Clever trick with the internalHandle fn!