Menu in StocksApp should animate out

sky/examples/stocks-fn/lib/stock_app.dart

Index: sky/examples/stocks-fn/lib/stock_app.dart
diff --git a/sky/examples/stocks-fn/lib/stock_app.dart b/sky/examples/stocks-fn/lib/stock_app.dart
index 1e59d3fdcf45cd82d442dc977b2fc83d89bd5f40..8eacdc827fb1579d0499ab11104198b211075bf3 100644
--- a/sky/examples/stocks-fn/lib/stock_app.dart
+++ b/sky/examples/stocks-fn/lib/stock_app.dart
@@ -145,8 +145,11 @@ class StocksApp extends App {
           // TODO(abarth): We should close the menu when you tap away from the
           // menu rather than when you tap on the menu.
           setState(() {
-            _menuController.close();
-            _menuController = null;
+            _menuController.close().then((_) {
+              setState(() {
+                _menuController = null;
+              });
+            });
           });
         }
       ));