Refactor Navigator to put state in separate class, initial back button plumbing

sky/examples/widgets/navigation.dart

Index: sky/examples/widgets/navigation.dart
diff --git a/sky/examples/widgets/navigation.dart b/sky/examples/widgets/navigation.dart
index ca81a4fbd79721b53072a57450d499e1d4defa90..82e81f850208556a873e105ce58a860633a06dbf 100644
--- a/sky/examples/widgets/navigation.dart
+++ b/sky/examples/widgets/navigation.dart
@@ -65,8 +65,10 @@ List<Route> routes = [
 ];
 
 class NavigationExampleApp extends App {
+  NavigationState _navState = new NavigationState(routes);
+
   Widget build() {
-    return new Flex([new Navigator(routes: routes)]);
+    return new Flex([new Navigator(_navState)]);
   }
 }