Custom elements: built-in element extensions lose their `is` attribute

[barneycarroll]

As demonstrated here, a custom element that extends a built-in via the is attribute behaves strangely. The extension is recognised and the custom element is constructed as expected; but the is attribute that determined this behaviour is not present on the rendered element. Having been specified in hyperscript, I would expect it to remain.

[pygy]

As long as we deal with [is], we have a render bug in the update phase. is should be considered part of the element identity along with the tag name when diffing.