Also, add two uses of Key and Val that I missed earlier.
As before, direct writes to Down and Type remain in bimport.

Change-Id: I487aa975926b30092db1ad74ace17994697117c1
Reviewed-on: https://go-review.googlesource.com/21330Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>