Support overloading method signatures in object types

I want to do something like this in Flow

```
type DifferentOutputs = {
doThing(name: 'number'): number;
doThing(name: 'Promise'): Promise;
}

function f(d: DifferentOutputs) {
var n: number = d.doThing('number');
}
```

This code is now rejected by Flow.

This logic is actually possible in flow - but only in the declaration files. See : https://github.com/facebook/flow/blob/master/lib/dom.js#L292-L310

I am not sure how to write this logic into my own types, and why it's possible only in external declarations.