[dart:html] Disallow extending html elements.

This change removes the `.created` constructor in the html
class hierarchy. These classes now only offer public factory
constructors. As a result, developers will no longer be able
to extend these classes in their libraries.

This is a breaking change, however we don't expect there to
be as much use of this feature (extending html element
classes). In particular, the ability to extend html elements
was intended for web components.  Dart only ever supported
the 0.5 spec of web components, which is old and long
deprecated.  Previously, in Dart 3.0, we removed APIs used
to register custom elements from `dart:html`, so the ability
to extend html elements has not been useful since then.

For more details see #53264

CoreLibraryReviewExempt: ddc/dart2js-specific library.
Change-Id: I43d8c9ae99dc83545e70e1f3b9dfc9f1b274a5c7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/321741
Reviewed-by: Lasse Nielsen <[email protected]>
Reviewed-by: Srujan Gaddam <[email protected]>
Reviewed-by: Stephen Adams <[email protected]>
Commit-Queue: Srujan Gaddam <[email protected]>

Author: sigmundch

CHANGELOG.md

@@ -8,6 +8,17 @@
 
 - Added `Iterable.withIterator` constructor.
 
+#### `dart:html`
+
+- **Breaking change**: Native classes in `dart:html`, like `HtmlElement`, can no
+  longer be extended. Long ago, to support custom elements, element classes
+  exposed a `.created` constructor that adhered to the v0.5 spec of web
+  components. On this release, those constructors has been removed and with that
+  change, the classes can no longer be extended. In a future change, they may be
+  marked as interface classes as well.  This is a follow up from an earlier
+  breaking change in 3.0.0 that removed the `registerElement` APIs. See
+  [#53264](https://github.com/dart-lang/sdk/issues/53264) for details.
+
 ### Tools
 
 #### Analyzer