Your errors list JS line numbers. I didn't find it very difficult to mentally map the JS errors back to my Coffee source though - you get the same program structure and variable/method names before and after compile.
Aha, your answer is more concise, and it addresses FooBarWidget's misapprehension more directly. Yes, "the runtime" is just the browser javascript engine.
Let's say you found a bug in line 41 of my JS source here: https://github.com/dpritchett/chatbox/blob/master/public/cha...
Digging into the Coffee source shows the same short method with all the same identifiers and the same basic structure... just less boilerplate: http://dpritchett.github.com/chatbox/docs/chatbox.html#secti...