Widgets should be stateful before calling setState

sky/sdk/lib/widgets/widget.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
 import 'dart:sky' as sky;
 
 import '../base/hit_test.dart';
@@ skipping 398 matching lines @@
     assert(root != null);
     _sync(null, _slot);
   }
 
   void scheduleBuild() {
     setState(() {});
   }
 
   void setState(Function fn()) {
     assert(!_disqualifiedFromEverAppearingAgain);
-    _stateful = true;
+    assert(_stateful);
     fn();
     if (_isBuilding || _dirty || !_mounted)
       return;
 
     _dirty = true;
     _scheduleComponentForRender(this);
   }
 
   Widget build();
 
@@ skipping 376 matching lines @@
         if (target is Listener)
           target._handleEvent(event);
         target = target._parent;
       }
     }
   }
 
 }
 
 abstract class App extends Component {
+
+  // Apps are assumed to be stateful
+  App({ String key }) : super(key: key, stateful: true);
+
   void _handleEvent(sky.Event event) {
     if (event.type == 'back')
       onBack();
   }
 
   void didMount() {
     super.didMount();
     SkyBinding.instance.addEventListener(_handleEvent);
   }
 
@@ skipping 81 matching lines @@
     if (root.parent == null) {
       // we haven't attached it yet
       assert(_container.child == null);
       _container.child = root;
     }
     assert(root.parent == _container);
   }
 
   Widget build() => builder();
 }