For an unknown index into a tuple, Flow neglects potential void types resulting from out of bounds access

The tuple docs claim that "If Flow doesn’t know which index you are trying to access it will return all possible types," but Flow neglects the possible void type:

```js
declare var p: [number];
declare var n: number;
(p[n]: number); //ng, but Flow passes.
(p[n]: number | void); //ok
```