This is in preparation for later changes.

Change-Id: I2b9b77a782cf65a2fcec5e700ec6bb8b1476f6b5
Reviewed-on: https://go-review.googlesource.com/c/142882
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>