add support for polymer mixins

[sigmundch]

for example core-focusable is a mixin in JS, so we could create a mixin class for it in Dart. Then users can mix the Dart class to get the extra API for the JS mixin.

[sigmundch]

It seems we need a couple things:

• request changing the pragma for mixins to be more explicit @element Polymer.CoreFocusable seems incorrect for this.
• add support to parse the .js file as well (currently we only parse HTML files), or see if this could be inlined into the HTML file in their repo (since they do that for every other element)

[jakemac53]

see googlearchive/core-focusable#5