trans.rs assertion error compiling code

I've reduced this down to about bare minimum:

```
use std;

native mod foo {
    fn bar(x: int);
}

obj new(i: int) {
    fn f() {
        str::as_buf("", {|_x| foo::bar(i) });
    }
}
```

Errors out with:

```
rust: upcall fail 'Assertion cx.fcx.llobjfields.contains_key(did.node) failed', ../src/comp/middle/trans.rs:2679
rust: domain main @0x102029600 root task failed
```
