[dart-announce] Breaking change #49536: remove document.registerElement

['Siggi Cherem' via Dart Announcements]

We'd like to do a small clean up in dart:html to deprecate and remove the `document.registerElement` API. This API is based on an old and outdated v0.5 web components specification and it is not supported by modern browsers today. 

See https://github.com/dart-lang/sdk/issues/49536 for more details, and let us know if you have any concerns or questions.

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