iOS builds have been broken due to missing main executable. Xcode
doesn't complain and buildling the target .app bundle even if a
referenced objects doesn't exist.
Fixing .app builds and adding a unit test.
Change-Id: Ie9aec6c8c982a96c10bf07344e44bd501fcabd19
Reviewed-on: https://go-review.googlesource.com/12492
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
