>>>
void main() {
  var controller = new StreamController<T>(sync: true, onCancel: () {
    return Future.wait(operationSet.map((operation) => operation.cancel()));
  });
}
<<<
void main() {
  var controller = new StreamController<T>(
    sync: true,
    onCancel: () {
      return Future.wait(operationSet.map((operation) => operation.cancel()));
    },
  );
}