add mixin support

fixes https://github.com/dart-lang/custom-element-apigen/issues/15
I see.

I think just filing a bug here:
https://github.com/dart-lang/www.dartlang.org/issues and setting the label "StyleGuide"

The closest issue I could find was this: https://github.com/dart-lang/www.dartlang.org/issues/819, but I didn't see any conclusion from it.