Parser doesn't allow optional arguments to be followed by required arguments

[domenic]

See https://github.com/whatwg/webidl/issues/1214. Apparently this restriction used to exist in 2013, but was removed in https://github.com/whatwg/webidl/commit/97499d48855a9f9012fe145d49b83ccd71d1738b.

(I got this error via Bikeshed, but I am assuming it's because of widlparser; my apologies if it's Bikeshed at fault instead. /cc @tabatkins)

[tabatkins]

Nah this is a widlparser issue. Opened #81 to fix it.