Enhancement request: js.Promise<Void> (or similar)

Separated this out of the conversation in #6790:

As long as we're thinking about js.Promise, it'd be nice if you could have a `Promise` (or just Promise), where resolve() and then(callback) expect no arguments. I know it's features like this that balloon the complexity, but I'm just saying -- it'd be nice. Otherwise I'm forced to write ugly crufty code like:

```
new Promise
resolve(null);
.then(function(ignore) { });
```

Cludgy Dynamic, useless null, and empty ignore -- all terrible conventions.

Maybe it'd be possible with abstract EmptyPromise (Promise)?

Maybe @elsassph / @kevinresol have some thought?

@jdonaldson commented:

> Here's some typescript folks struggling with the same issue:
> Microsoft/TypeScript#8516
> Microsoft/TypeScript#3356
> Microsoft/TypeScript#4260
> DefinitelyTyped/DefinitelyTyped#6091