//line directives with filenames are not correctly parsed.

As a result, https://github.com/golang/go/blob/master/test/fixedbugs/issue18149.go test fails with:

```
Error: got /private/var/folders/tw/kgz4v2kn4n7d7ryg5k_z3dk40000gn/T/bar.go:123; want /foo/bar.go:123
```

Not an extremely important bug to fix. Issue is for tracking purposes.