See CL 9962 for the rationale.

Change-Id: I73c714fce258430eea1e61d3835f5c8e9014ca1f
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/9925Reviewed-by: Ian Lance Taylor <iant@golang.org>