Dunno about sweet.js, but in Lisps, it's usually source order. E.g., read the next token, is it at the head of the list and names a macro, if so, call the macro

Dunno about sweet.js, but in Lisps, it's usually source order. E.g., read the next token, is it at the head of the list and names a macro, if so, call the macro function, and pass in the code as the param, take the output and replace the macro form with the expanded version, resume reading...