add a dart paper sample

@ussuri, add a dart web app using polymer / paper sample.

The sample runs in both Chrome and Dartium. I need to do explicit component registration (`upgradePaperToast()`), which is not normal for Dart polymer. Not sure if there's a way to avoid this.
It would be good to try to do this in a Polymer way, meaning `on-click="{{showToast}}"` in the HTML. Not sure if that would work, given that this code is on the Polymer/non-Polymer boundary.