A rewrote my original comment because it was ambiguous:

> checkout/subform.js should be precompiled by virtue of checkout/checkout.js being precompiled

It might be we need to add `javascript_include_tag 'checkout/checkout.js'` here instead. It might also be that we need to precompile `checkout/subform.js` so it's available standalone. I'm not sure and will need to check.