[webidl] Sort out when dictionaries should have default values (#76)

Right now optional dictionary arguments in trailing position have a default value (empty dictionary) but other optional dictionaries (non-trailing arguments, dictionary members) do not.

This allows spec authors to create APIs in which undefined and empty dictionary have different behavior, which seems suboptimal.

So I'd like to propose that optional dictionaries always have empty dictionary as default value.  What that means in practice is not clear.  I guess they should use `null` as the default value.  Sadly that's observably different from using `{}` if someone spews things on `Object.prototype`...

Anyway, if we do that, then we have one problem: dictionary-to-JS conversion can give ugly results (as described in https://bugzilla.mozilla.org/show_bug.cgi?id=1226475#c0 for example).

To solve that, I propose the following:

1. A dictionary is "elidable" (better-naming-wanted) if it is either empty or only has members whose values are themselves elidable dictionaries.
2. When converting a dictionary to a JS value, skip over members whose values are elidable dictionaries, just like we skip over members that are not present.

For dictionaries that have default values that will still cause them to appear (with those default values), but I can live with that, I think.

@domenic/@heycam Thoughts?  

---
Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/76