Implement required dictionary defaulting

[annevk]

See whatwg/webidl#750. Presumably this will depend on plinss/widlparser#39.

[plinss]

See plinss/widlparser#39 (comment) notes about type resolution

[tabatkins]

And Bikeshed is now updated to widlparser trunk, so the parser should work.

[plinss]

@tabatkins lmk if you want to work on adding the type resolution to support the error detection, happy to brainstorm on that...

[tabatkins]

Yeah, I've been having discussions with @foolip about Reffy integration (which does similar scans over its own spec list, and extracts WebIDL); if Shepherd wants to handle it all that's cool with me. I'll hit you on IRC.