Incorrect span for method lookup failure

For example, if I change the name of `::std::StrSlice::rfind` I get:

```
.../rust/src/libstd/path.rs:106:22: 106:36 error: type `&&str` does not implement any method in scope named `rfind`
.../rust/src/libstd/path.rs:106                 match f.rfind('.') {
                                                      ^~~~~~~~~~~~~~
```

The Span should probably just point at the `rfind` itself.
