RFC: More flow-friendly thenable checking.

[leebyron]

This replaces the isThenable boolean check with a "getIfPromise" function. It's essentially the same, but rather than returning boolean, it's the identify function when it would have returned true, and returns void otherwise. That lets us give it a better type signature that Flow can do more with.

Open to feedback on this one, and open to bikeshedding on the name of this new function.

[wincent]

I like that this consolidates the any cast to a single location. The getIfPromise name seems all right. Other random ideas that cross my mind are maybePromise, or a safePromise wrapper which you would use like this:

safePromise(maybeAPromise).then(() => {
  // Will only be called if `maybeAPromise` had a `then` method...
});

(Costs: extra function creation, and attendant cost of capturing a closure.)

[leebyron]

Yeah I'd like to avoid the costs of doing so, plus in a lot of those cases we're actually returning early if it's a promise, so that might not work.

I wish javascript supported if-let conditions, that would drop one LOC for each use.