To prevent slice growths with append operations.

Change-Id: Icdb745b23cc44dfaf3e16746b94c06997f814e15
Reviewed-on: https://go-review.googlesource.com/23784Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>