Consider semantic changes to WebIDL to make suitable for ECMAScript

[littledan]

If we were going to use WebIDL for the ECMAScript specification, we'd need a few more features, such as:

• Non-enumerable methods
• (possibly) declarations of own instance data properties
• ... (there is probably more; I will come back to edit this issue to fill more in)

[bzbarsky]

Non-enumerable methods

This came up in #226 (and in general there is other discussion in there that may be relevant here).

(possibly) declarations of own instance data properties

https://www.w3.org/Bugs/Public/show_bug.cgi?id=27346 discusses that; looks like it was not migrated to a github issue.

[domenic]

• Methods and getters that don't brand check

[TimothyGu]

• Class strings of prototype objects

[domenic]

The latter is fixed by #357, FWIW.

[littledan]

This is an easy one, but we'll want more types, such as to express ToIndex.