Abstract out the AppView logic in fn so that it can also be used in non-pure-fn apps.

This lays the groundwork for using fn widgets in static environments.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1178703002

examples/stocks2/lib/stock_app.dart

@@ -46,8 +46,6 @@ class StocksApp extends App {
     _drawerController = new DrawerController(_handleDrawerStatusChanged);
   }
 
-  void syncFields(StocksApp source) { }
-
   bool _isSearching = false;
   String _searchQuery;