Upgrade to Polymer 0.13.0

Full credit for discovering the root cause goes to @rpaquay: thanks! Feel free to just cherry pick from here and land a PR in your name.
`startPolymer` is gone, but `initPolymer` is still needed. This should change to something like this:

```dart
main() => polymer.initPolymer();
@polymer.initMethod
init() {
  // move here what you currently have in main
}
```


Or:
```dart
main() => polymer.initPolymer().run(() {
  ...
});
```


Or:
```dart
export 'package:polymer/init.dart';

@polymer.initMethod
init() {
  ...
}
```