anchor.diff:    ///
     /// This is a stream which does nothing except sending a done event
     /// when it's listened to.
  +  ///
  +  /// Example:
  +  /// ```dart
  +  /// const stream = Stream.empty();
  +  /// stream.listen(
  +  ///   (value) {
  +  ///     throw "Unreachable";
  +  ///   },
  +  ///   onDone: () {
  +  ///     print('Done');
  +  ///   },
  +  /// );
  +  /// ```
     const factory Stream.empty() = _EmptyStream<T>;
   
     /// Creates a stream which emits a single data event before closing.
  
positive.code:   /// The returned stream is a broadcast stream if this stream is.
    /// If a broadcast stream is listened to more than once, each subscription
    /// will individually perform the `test`.
    Stream<T> where(bool test(T event)) {
      return new _WhereStream<T>(this, test);
    }
  
negative.code: /// and holds the callbacks used to handle the events.
  /// The subscription can also be used to unsubscribe from the events,
  /// or to temporarily pause the events from the stream.
  ///
  /// Example:
  /// ```dart
  /// final stream = Stream.periodic(const Duration(seconds: 1), (i) => i * i)
  ///     .take(10);
  ///
  /// final subscription = stream.listen(print); // A StreamSubscription<int>.
  /// ```
  /// To pause the subscription, use [pause].
  /// ```dart continued
  /// // Do some work.
  /// subscription.pause();
  /// print(subscription.isPaused); // true
  /// ```
  /// To resume after the pause, use [resume].
  /// ```dart continued
  /// // Do some work.
  /// subscription.resume();
  /// print(subscription.isPaused); // false
  /// ```
  /// To cancel the subscription, use [cancel].
  /// ```dart continued
  /// // Do some work.
  /// subscription.cancel();
  /// ```
  abstract class StreamSubscription<T> {
    /// Cancels this subscription.
    ///
  