Specifically Addstring, Addbytes and Symgrow.

Change-Id: Ia74093bfcf9f360bf223accbc8feef54a7f059c9
Reviewed-on: https://go-review.googlesource.com/29348
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>