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.
The latest-and-greatest way to declare the main Polymer method is:

```dart
@polymer.initMethod
void main() {
   ...
```

I don't think it's going to eliminate the need for explicit registration below, though: we're still have to do the same in CDE itself, otherwise it doesn't work.