Stream transforms in Dart are too complicated and seem to require the below over and over, if it's not generic. The file below should be useable in any Dart project to transform streams.

stream_transformer.dart

import 'dart:async';

/*
* https://api.flutter.dev/flutter/dart-async/Stream/Stream.eventTransformed.html
*
* <S, T>
*     S ==> Snapshot type (DocumentSnapshot, QuerySnapshot, et. al.)
*     T ==> The type to be returned (Event, List<EventMedia>, et. al.)
*
*  This should probably never need to change. It seems to work pretty well.
*
*  Pass instance of EventStreamTransformer<S, T> to stream.transform. Pass in a callback
*  function that will do the actual transformation. T will be the returned stream type
*  
*  Example:
*  return stream.transform(EventStreamTransformer<DocumentSnapshot, Event>(
*       (DocumentSnapshot documentSnapshot) {
*     Event event = EventSerializer.documentSnapshotFromFirestore(documentSnapshot);
*     event.id = documentSnapshot.documentID;
*     return event;
*   }));
* */
class EventStreamTransformer<S, T> extends StreamTransformerBase<S, T> {
  final Function _transform;

  EventStreamTransformer(this._transform);

  @override
  Stream<T> bind(Stream<S> stream) {
    return Stream<T>.eventTransformed(
      stream,
      (EventSink sink) => EventStreamSink<S, T>(sink, _transform),
    );
  }
}

class EventStreamSink<S, T> implements EventSink<S> {
  final EventSink<T> _outputSink;
  final Function _transform;

  EventStreamSink(this._outputSink, this._transform);

  @override
  void add(S snapshot) {
    T transformed = _transform(snapshot);
    _outputSink.add(transformed);
  }

  @override
  void addError(Object error, [StackTrace stackTrace]) {
    _outputSink.addError(error, stackTrace);
  }

  @override
  void close() {
    _outputSink.close();
  }
}