rustc error references the old '~' syntax for boxes

``` rust
struct A;

impl A {
    fn foo(&mut self) {
    }
}

fn main() {
    let a = box A;

    a.foo();
}
```

Gives:

```
test.rs:11:5: 11:6 error: cannot borrow immutable dereference of `~`-pointer `*a` as mutable
test.rs:11     a.foo();
```
