The code has to be explicitly marked as asm.js, using a pragma similar to ES5 strict mode. This way if the code fails to validate, the errors can be reported to the browser's developer tools.

As for debugging, the story is the same as Emscripten. I believe there's plenty of work to do to make it better, but it's no different than the existing state of affairs. All we're doing is formalizing existing practice so that engines can capitalize on it and optimize better.