Type inference not working with ES6 "export default"

Here is a simple way to reproduce:

``` js
//A.js
export default class A {
}
```

``` js
//B.js
import A from './A'

export default class B extends A {
}
```

Compilation will generate a warning:
`/src/B.js: WARNING - Bad type annotation. Unknown type module$$src$A.default`

Current workaround: use non-default exports.