I don't think Google prefer either, YouTube was recently migrated to Polymer, They use Angular(Dart) from Adwords. almost all other projects use vanilla JavaScript(with closure compiler)
The Polymer philosophy appears to be more of "make everything a custom element", i.e. up to the entire app (e.g. "<youtube-app/>", "<comments-section video-id=''/>", "<related-videos video-id=''/>", etc.).