Widgets should be stateful before calling setState

sky/sdk/lib/widgets/widget.dart

Index: sky/sdk/lib/widgets/widget.dart
diff --git a/sky/sdk/lib/widgets/widget.dart b/sky/sdk/lib/widgets/widget.dart
index 9b4001eff9292b7c84cd71edd81242dc4b024860..4ad23dc9787f4c62beadcbf2ab0ed9c1c8f00c28 100644
--- a/sky/sdk/lib/widgets/widget.dart
+++ b/sky/sdk/lib/widgets/widget.dart
@@ -416,7 +416,7 @@ abstract class Component extends Widget {
 
   void setState(Function fn()) {
     assert(!_disqualifiedFromEverAppearingAgain);
-    _stateful = true;
+    assert(_stateful);
     fn();
     if (_isBuilding || _dirty || !_mounted)
       return;
@@ -813,6 +813,10 @@ class WidgetSkyBinding extends SkyBinding {
 }
 
 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();