I don't know why these errors occur. Ignore them to avoid breaking the
build.

Updates #22019

Change-Id: Ia048e6d9b928e8e237b311ff3a364e7a23af4aa4
Reviewed-on: https://go-review.googlesource.com/86000
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>