"Type 'void' not found" returned from CFE when using void as a type parameter to a mixin

[ds84182]

Dart version: Dart VM version: 2.1.0-dev.9.4 (Unknown timestamp) on "linux_x64"

Error message: lib/snakeway/snakeway.dart:1: Error: Type 'void' not found.

mixin _Batch<I, V, T, B> {
  final Map<I, B> _batch = {};
  bool _scheduled = false;

  B makeBatch();
  Future<T> addToBatch(B batch, V param);
  void handleBatch(Map<I, B> batch);

  Future<T> get(I id, [V param]) {
    if (!_scheduled) {
      _scheduled = true;
      Timer.run(() {
        _scheduled = false;
        final copy = Map.of(_batch);
        _batch.clear();
        handleBatch(copy);
      });
    }

    return addToBatch(_batch[id] ??= makeBatch(), param);
  }
}

// Changing these `void`s below to `Null` made the error go away
abstract class _SimpleBatch<I, T> with _Batch<I, void, T, Completer<T>> {
  @override
  Completer<T> makeBatch() => Completer.sync();

  @override
  Future<T> addToBatch(Completer<T> completer, void param) => completer.future;
}

Analyzer reports no errors.