go.tools/go/types: test case for issue 6413
go.tools/go/types: test case for issue 6413

Fixes issue 6413.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/13840047

tmc/mirror-go.tools,tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools,t-yuki/go.tools.old,t-yuki/go.tools.old