Not seeing any SyntaxErrorNodes

[andrewdep]

The language.js readme says that the parser will produce SyntaxErrorNodes on parse errors, but not only do I not see any in the output, I don't even see any SyntaxErrorNode code in the generated parser. Either I'm doing something wrong or this isn't yet implemented?
If I have a syntax error in my source, the parser produces a cst with nothing in it (except a root #document node).