This error only affects the compilation of the standard library,
but I discovered that if you import "notexist" from the standard
library then you get both an error about notexist not existing
and an error about notexist being a non-standard package
(because the non-existant package is in fact not a standard package).
Silence the second error.

Change-Id: Ib4c1523e89844260fde90de3459ec1e752df8f25
Reviewed-on: https://go-review.googlesource.com/19383
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>