Rename App to MaterialApp

MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

examples/game/lib/main.dart

diff --git a/examples/game/lib/main.dart b/examples/game/lib/main.dart
index 9150ab9..098f3eb 100644
--- a/examples/game/lib/main.dart
+++ b/examples/game/lib/main.dart
@@ -81,7 +81,7 @@
   int _lastScore = 0;
 
   Widget build(BuildContext context) {
-    return new App(
+    return new MaterialApp(
       title: 'Asteroids',
       theme: _theme,
       routes: {