Instead of making a runtime call, compare types and values.

Change-Id: Id302083d5a6a5f18e04f36f304f3d290c46976ad
Reviewed-on: https://go-review.googlesource.com/26660
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>