Cross-crate unit structs are broken

a.rs:

``` rust
pub struct Foo;
```

b.rs:

``` rust
extern mod a;

fn main() {
    let foo = a::Foo;
}
```

```
b.rs:4:14: 4:20 error: unresolved name `a::Foo`.
b.rs:4     let foo = a::Foo;
                     ^~~~~~
error: aborting due to previous error
```
