Hover information over `export default function` does not work

If you have a file with the following contents:

```
export default function(a: string, b: number) {
  return Promise.resolve(a + b);
}
```

Then there is no way to see function signature through hovering. Basically, because it has no name? It would be nice to get the hover information over the `export` or `default` keyword :-)
