interfaces and type aliases behave differently on optional fields

interface `IteratorResult` from `lib/core.js`

``` js
interface IteratorResult<T> {
  done: boolean;
  value?: T;
}

// error: property value Property not found in IteratorResult
var x: IteratorResult = {done: true};
```

conversely this works fine:

``` js
type IteratorResult<T> = {
  done: boolean;
  value?: T;
};

// ok
var x: IteratorResult = {done: true};
```
