ICE when `Add` trait is missing the `add` method

if the method is missing, rustc panics due to an unwrap on a `None`:

``` rust
#![feature(no_core, lang_items)]
#![no_core]

#[lang="sized"]
trait Sized {}

#[lang="add"]
trait Add<T> {}

impl Add<i32> for i32 {}

fn main() {
    let x = 5 + 6;
}
```
