ICE when constructing an array whose size is in an external crate

``` rust
    extern crate n; // contains only: pub static N: uint = 24;
    pub struct X { n: [u8,..24] }
    fn new() -> X { X { n: [0,..n::N] }}
    fn main() {}
```

RefCell<T> already borrowed in middle::astencode::decode_side_tables::closure.
