Make promise selection logic clearer

[sophiebits]

No description provided.

[sophiebits]

@fisherwebdev Can you review?

[fisherwebdev]

We revised this in #1480. The problem here is that the indexes for promiseIndexes and the indexes for _promises are not 1:1.

[sophiebits]

Before #1480, it looks like waitFor([7, 3]) would return [_promises[0], _promises[1]]. With the current committed code it returns [_promises[3], _promises[7]] and with my change here it should just return [_promises[7], _promises[3]].

[fisherwebdev]

Yep you're right.

[fisherwebdev]

I advised Paul O to accept.

[sophiebits]

I'll take that as approval and just merge it myself. :)