We're actually not using the Polymer Web Components shim- it attempts to simulate Shadow DOM by overriding DOM Methods(!), which didn't seem necessary.
Yep. The web components aspect is the most interesting aspect of this. It would have been useful to put that in the post title (until a mod reverts it).
Underlying Polymer's opinionated framework, there is a set of polyfills for the emerging Web Components standards - both Mozilla's X-Tag library and Polymer use these polyfills as the basis of their offerings.