This fixes the build with go1.4 during bootstrap.

Change-Id: I35fa1dc3d877d31d5ffe515f5f19188f4a8b77f4
Reviewed-on: https://go-review.googlesource.com/c/143377
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>