breaking change #53264: HtmlElement cannot be extended

[Siggi Cherem]

We'd like to do follow-up cleanup in dart:html to remove the ability to extend "native" classes like HtmlElement. This functionality was there to support our old and outdated v0.5 web components implementation, which was previously deprecated and removed since Dart 3.0.

See https://github.com/dart-lang/sdk/issues/53264 for more details. Let us know if you have any concerns or questions.