gosrc: fix IsDir implementation
gosrc: fix IsDir implementation

Fixes golang/gddo#504

Change-Id: Ic17c2b10690cb156678759d1e5bb1632828a34e2
Reviewed-on: https://go-review.googlesource.com/59191
Reviewed-by: Dmitri Shuralyov <0c2a7749d909c8353e59c3f9c6a4d19a5ce000c1@gmail.com>

golang/gddo,segmentio/gddo,segmentio/gddo,golang/gddo,segmentio/gddo,golang/gddo,segmentio/gddo,golang/gddo