a91cc6c701
document.registerElement is going away soon, and dart2js never implemented window.customElements.define. Replace the inheritance with composition for each FooElement. When used to create a DOM tree, replace `new FooElement()` with `new FooElement().element`. In CSS, replace each custom element selector with a class selector, i.e. `foo-element` with `.foo-element`. Bug: https://github.com/dart-lang/sdk/issues/34107 Change-Id: Id5ab5b6104a6165a8ad3b01d73ee9d76e46024ed Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/101061 Reviewed-by: Ben Konyi <bkonyi@google.com> Commit-Queue: Ryan Macnak <rmacnak@google.com>