   <h2><span>Implementation</span> </h2>
      <pre class="language-dart"><code class="language-dart">Stream&lt;E&gt; asyncExpand&lt;E&gt;(Stream&lt;E&gt; convert(T event)) {
  StreamController&lt;E&gt; controller;
  StreamSubscription&lt;T&gt; subscription;
  void onListen() {
    assert(controller is _StreamController ||
        controller is _BroadcastStreamController);
    final _EventSink&lt;E&gt; eventSink = controller as Object;
    subscription = this.listen((T event) {
      Stream&lt;E&gt; newStre