Feature request: warn when behaviors clash with each other or with mixed-in elements

[rictic]

As an element author, when I use behaviors I would like to get a warning when there is an accidental clash between the behaviors or between any of my behaviors and my element's definition.

I also would like to be able to annotate that a property or a behavior should intentionally be overridden, perhaps with the @overrides or @abstract jsdoc syntax.