Calling method on closure trait object leads to 'source trait is private' error

``` rust
#![feature(unboxed_closures, unboxed_closure_sugar)]

fn main() {
    let task: Box<|: int| -> int> = box |: x| x;
    task.call_once((0i, ));
}
```

```
<anon>:5:5: 5:27 error: source trait is private
<anon>:5     task.call_once((0i, ));
             ^~~~~~~~~~~~~~~~~~~~~~
```

(Adding a `use std::ops::FnOnce` hits #16599.)
