[heycam/webidl] Update the ecosystem to constructor methods (#778)

See #636, #700.

Tools
=====
* [x] [webidl2.js](https://github.com/w3c/webidl2.js) - https://github.com/w3c/webidl2.js/pull/300
* [ ] [idlharness.js](https://github.com/w3c/respec)
* [ ] [widlparser](https://github.com/plinss/widlparser)
* [ ] [bikeshed](https://github.com/tabatkins/bikeshed)
* [ ] [respec](https://github.com/w3c/respec)

Specs
=====
TBD

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