Give an error for a mismatch passing ... to ...T or vice-versa.
Give an error for a mismatch passing ... to ...T or vice-versa.

R=iant
https://golang.org/cl/203052

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