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

[domenic]

See whatwg/webidl#1214. Apparently this restriction used to exist in 2013, but was removed in whatwg/webidl@97499d4.

(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.