[webcomponents] Add drawbacks to `is=` proposal (#117)

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/117

-- Commit Summary --

  * Add drawbacks to `is=` proposal

-- File Changes --

    M proposals/Type-Extensions.md (51)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/117.patch
https://github.com/w3c/webcomponents/pull/117.diff

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https://github.com/w3c/webcomponents/pull/117