[whatwg/dom] Remove incorrect customized built-in element checks (#373)

createElement/createElementNS disallowed upgrading of customized
built-in elements, incorrectly. Instead we should allow { is } to be
supplied and even if there is no corresponding definition now, it will
be upgraded later.

Fixes https://github.com/w3c/webcomponents/issues/608.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/373

-- Commit Summary --

  * Remove incorrect customized built-in element checks

-- File Changes --

    M dom.bs (14)

-- Patch Links --

https://github.com/whatwg/dom/pull/373.patch
https://github.com/whatwg/dom/pull/373.diff

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https://github.com/whatwg/dom/pull/373