This reverts commit eee727d0
(https://golang.org/cl/29113)

The " (.go files ignored due to build tags)" error message is not
always accurate.

Fixes #18396
Updates #17008

Change-Id: I609653120603a7f6094bc1dc3a83856f4b259241
Reviewed-on: https://go-review.googlesource.com/34662
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>