Allows safely eliminating more direct uses of Type's Type and Down
fields.

Passes toolstash -cmp.

Change-Id: I5c17fe541a0473c3cd2978d8314c4ab759079a61
Reviewed-on: https://go-review.googlesource.com/20541
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>