Built-in element extension in V2.

[PaulHMason]

I'm probably going to be shot for this :)

Is there currently any guidance on how to extend built-in elements? There's a whole thread about "is" with no apparent resolution here: WICG/webcomponents#509

Unfortunately, it's kind of a pressing issue in certain instances (particularly table child elements, which I'm bumping my head against now).

A Polymer only solution isn't ideal, but it looks like the only practical one, if possible.

[web-padawan]

See this comment for explanation.

[nazar-pc]

Also see #3925

[kevinpschaaf]

Closing as a dupe of #3925, which captures the current state of the issue.