Handle some params that should be optional.

This has some false positives, but it's an improvement. Man, what I
wouldn't give right now for a high quality set of IDLs. I'm finding out
that WebCore's IDLs are meant more for API *implementors* rather than
API *specifiers*.

Mozilla's new WebIDLs look amazing, they're incomplete, but possibly for
Haxe 3.2: http://mxr.mozilla.org/mozilla-central/source/dom/webidl/