Should be something like `.transform(new RiskGameEventTransformer())` with `RiskGameEventTransformer` like
```Dart
class RiskGameEventTransformer implements StreamTransformer<PlayerEvent, Object> {
  // Some generic types ommitted for brevety.
  Stream bind(Stream stream) => new Stream<String>.eventTransform(
      stream,
      (EventSink sink) => new RiskGameEngine(sink));
}
```