all: fix vet errors

It would be nice to run these alongside the tests. There is only one
vet error left in the x/build tree, an unreachable return next to a
TODO.

Change-Id: Ic31d96cdee5bb0f6b9795237ff3b743a4b76e3cf
Reviewed-on: https://go-review.googlesource.com/36898
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
7 files changed