Avoid producing // in an import file name.
Avoid producing // in an import file name.

R=iant
https://golang.org/cl/3186041

golang/gofrontend,golang/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend