Re: Cancellation architectural observations

> function doLater(callback, token) {
>   var handle = setImmediate(callback);
>   token.register(() => clearImmediate(handle));
> }
>
> var cts = new CancellationTokenSource();
> doLater(..., cts.token);
> cts.cancel();
>

I'm not really clear about the ordering properties of setImmediate relative
to the promise microtask queue, but:

    function doLater(callback, cancelPromise) {
        let handle;
        cancelPromise.then(_=> clearImmediate(handle));
        handle = setImmediate(callback);
    }

This should work if setImmediate uses either the same MT queue as promises
or a lower-priority queue.