The `registerElement` APIs in `dart:html` are legacy APIs based on a
deprecated Web Components v0.5 specification. These methods don't work
on modern browsers and can only be used with a polyfill.
The latest Web Components specification is supported indirectly via
JSInterop and doesn't have an explicit API in the `dart:html` library.
This change marks these APIs as deprecated. We intend to remove them in
the future (see https://github.com/dart-lang/sdk/issues/49536)
Fixes https://github.com/dart-lang/sdk/issues/48973
Change-Id: I2e96d36e95c9971b59cde80bc4da49b63d12b17c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/252840
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>
