Let firstOrNull extension method wait for cancel to complete. (dart-archive/async#196)

Author: lrhn

pkgs/async/lib/src/stream_extensions.dart

@@ -43,19 +43,15 @@ extension StreamExtensions<T> on Stream<T> {
  /// completed with `null`.
  Future<T?> get firstOrNull {
  var completer = Completer<T?>.sync();
- final subscription = listen(null);
- subscription
- ..onData((event) {
- subscription.cancel();
+ final subscription = listen(null,
+ onError: completer.completeError,
+ onDone: completer.complete,
+ cancelOnError: true);
+ subscription.onData((event) {
+ subscription.cancel().whenComplete(() {
  completer.complete(event);
- })
- ..onError((Object error, StackTrace stackTrace) {
- subscription.cancel();
- completer.completeError(error, stackTrace);
- })
- ..onDone(() {
- completer.complete(null);
  });
+ });
  return completer.future;
  }
 }