This will reduce duplication when evacuate is specialized.

Change-Id: I34cdfb7103442d3e0ea908c970fb46334b86d5c4
Reviewed-on: https://go-review.googlesource.com/56934
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>