title: Flutter: yield in listener for Stream
body: <p>How I can provide the object <code>bar</code> from listener (line 4) to given Stream?</p>
  <pre><code>Stream&lt;Foo?&gt; listenAnswerForQuestion(String questionId) async* {
      yield getAnswerForQuestion(questionId);
  
      _repository.addListener(() async* {
        final bar = getAnswerForQuestion(questionId);
        yield bar;
      });
    }
  </code></pre>
  <p>Thanks for help in advance!</p>