Chaining Arc<RefCell<..>> confuses borrowck.

http://is.gd/rNjThX

```
use std::sync::Arc;
use std::cell::RefCell;

struct A {
    a : Arc<RefCell<i32>>
}

struct B {
    b : Arc<RefCell<A>>
}

impl B {
    fn is_foo(&self) -> bool {
        *self.b.borrow().a.borrow() == 2
    }
}
```

This should compile, but it does not. Breaking the chain is a workaround.
