title: notifyListeners in Flutter not working correctly
body: <p>I have a Model that calls notifyListeners and a <code>ScopedModelDescendant</code> widget in a build, even though it 100% calls the notifyListeners(), it doesn't update the build.</p>
  <p>I looked into it and it seems the Model doesnt have anything in the _listeners.</p>
  <pre><code>class MyModel extends Model {
  
    void notify() {
      notifyListeners();
    }
  
  }
  
  class Main extends StatefulWidget {
  
    _MainState createState() =&gt; _MainState();
  
  }
  
  class _MainState extends State&lt;StatefulWidget&gt; {
  
    Widget build(BuildContext context) {
      return ScopedModel&lt;MyModel&gt;(
        model: MyModel(),
        child: ScopedModelDescendant&lt;MyModel&gt;(
          builder: (context, child, model) {...}
        ),
      );
    }
  }
  </code></pre>
  <p>So as expected when I call notify, whatever is in the {...} should get rebuild, but it doesnt.</p>