Non-main packages are built in an earlier code path than main packages. Add the ios build tag before that early code path to ensure packages that expect that tag successfully compiles. Fixes golang/go#25944 Change-Id: Ida15475109373127dde024037e9787c76b32ee0b Reviewed-on: https://go-review.googlesource.com/119555 Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>