Fix type assertions for global variables.  Fix unsafe.Pointer checks for type assertions.  Give an error for impossible type assertions.
Fix type assertions for global variables.  Fix unsafe.Pointer
checks for type assertions.  Give an error for impossible type
assertions.

R=iant
https://golang.org/cl/2164043

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