[webidl] All supported property names should be enumerable or none of them (#82)

Currently in the platform, the only interface with [unenumerable](http://heycam.github.io/webidl/#dfn-unenumerable) property names is [NamedNodeMap](https://dom.spec.whatwg.org/#dom-htmlcollection-item). In that interface, they are *all* unenumerable.

WebIDL should reflect that and forbid mixing unenumerable and enumerable property names.

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Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/82