dart:html: document.getElementById('dialog') returns UnknownElement instead of DialogElement

[DartBot]

This issue was originally filed by alexe.kaigorodov@gmail.com

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following code:

const String htmlEL = r'''
<dialog id="d1">
  <p>foobar</p>
  <button>OK</button>
</dialog>
''';

void main() {
  document.body.appendHtml(htmlEL);
  var d1 = document.getElementById('d1');
  print("d1 is ${d1.runtimeType}");
}

prints

d1 is UnknownElement

[sgjesse]

Added Library-Html, Triaged labels.

[alan-knight]

In 1.8 it returns HtmlElement rather than UnknownElement but it still seems incorrect.

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Removed Priority-Unassigned label.
Added Priority-Medium, Accepted labels.