Simplifies the code somewhat and allows removing Param.Field.

Passes toolstash-check.

Change-Id: Id854416aea8afd27ce4830ff0f5ff940f7353792
Reviewed-on: https://go-review.googlesource.com/99336
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>