I like the idea, but I am not a fan of the compile-step.<p>If there was an alternative implementation of JavaScript with labels used only in this manner (pre-defined meaning, no usage otherwise) they would actually be a good practice.
author here, this was my easter hack. You might find the online demo a bit more accessible: <a href="http://codemix.github.io/contractual/try.html" rel="nofollow">http://codemix.github.io/contractual/try.html</a>