`Constants.register` is required because RubyMotion only allows constants which are declared at compile-time to be found at run-time. So, in general, if you do a `rake` and type a constant into the terminal which is not directly in your codebase, like "UIBarButtonSystemItemTrash", it'll thrown a `NameError`. Thus things like `Constant.get` will break.

The reason the tests pass is because, in your test spec code, you've included all the constants you're comparing against. If you try doing a `BW::UIBarButtonItem.build` on the console, it'll throw an error.