Compilation error with Box<FnOnce(uint) -> uint>

``` rust
fn action(cb: Box<FnOnce(uint) -> uint>) -> uint {
    cb(1)
}

pub fn main() {
    println!("num: {}", action(|u| u));
}
```

output:

```
$ rustc fnonce.rs
fnonce.rs:1:25: 1:26 error: expected `,`, found `(`
fnonce.rs:1 fn action(cb: Box<FnOnce(uint) -> uint>) -> uint {
```

/cc @pcwalton 
