This is necessary once js2coffee wants to use Esprima 3.0.
In a FunctionDeclaration or FunctionExpression, the defaults array to store the default parameter values is gone. Instead, each value is available in each parameter via an AssignmentExpression. For details and an example, see jquery/esprima#1081.
Note that my CoffeeScript skill is at the n00b level, so I'm not sure I could come up with a patch. But I'd be happy to help otherwise!
thanks so much for keeping me updated on this! will squeeze in some time to grok the issue and figure out what to do then.