No need to keep as Nodes when they're all Types anyway.

Change-Id: I8157914ba5b09cadf2263247844680a60233a0f2
Reviewed-on: https://go-review.googlesource.com/37886
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>