[webcomponents] Add "Constructor Dmitry" proposal (#294)

This is an attempt to outline what I consider the "current state of the art" in custom elements proposals. It combines the Dmitry proposal, which allows author-defined classes to be used instead of generating them from a `{ prototype }` bucket, with the constructor-call trick discovered at the F2F. It also updates lifecycle callbacks to call attribute changed during creation, and to use symbols instead of string names.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/294

-- Commit Summary --

  * Add "Constructor Dmitry" proposal

-- File Changes --

    A proposals/Constructor-Dmitry.md (240)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/294.patch
https://github.com/w3c/webcomponents/pull/294.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/pull/294