`Self` fails to implement current trait in a default method

``` rust
trait Foo {
    fn foo(&self) {
        bar(self)
    }
}

fn bar<F: Foo>(_: &F) {}

fn main(){}
```

```
dm2.rs:3:8: 3:11 error: failed to find an implementation of trait Foo for Self
dm2.rs:3         bar(self)
                 ^~~
```

Presumably `Self : Foo` in a default method on `Foo`.

(Part of #2794.)
