Add back/forward history to navigation

R=abarth@chromium.org, abarth, hixie

Review URL: https://codereview.chromium.org/1181773006.

examples/widgets/navigation.dart

@@ -4,28 +4,69 @@
 
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/navigator.dart';
+import 'package:sky/widgets/transition.dart';
 import 'package:sky/widgets/raised_button.dart';
 
 List<Route> routes = [
   new Route(
-    name: 'safety',
-    builder: (navigator) => new RaisedButton(
-      child: new Text('PRESS FORWARD'),
-      onPressed: () => navigator.pushNamedRoute('adventure')
+    name: 'home',
+    builder: (navigator) => new Container(
+      padding: const EdgeDims.all(20.0),
+      decoration: new BoxDecoration(backgroundColor: const Color(0xFFCCCCCC)),
+      child: new Block([
+        new Text("You are at home"),
+        new RaisedButton(
+          key: 'b',
+          child: new Text('GO SHOPPING'),
+          onPressed: () => navigator.pushNamed('shopping')
+        ),
+        new RaisedButton(
+          key: 'a',
+          child: new Text('START ADVENTURE'),
+          onPressed: () => navigator.pushNamed('adventure')
+        )
+      ])
+    )
+  ),
+  new Route(
+    name: 'shopping',
+    builder: (navigator) => new Container(
+      padding: const EdgeDims.all(20.0),
+      decoration: new BoxDecoration(backgroundColor: const Color(0xFFBF5FFF)),
+      child: new Block([
+        new Text("Village Shop"),
+        new RaisedButton(
+          key: 'a',
+          child: new Text('RETURN HOME'),
+          onPressed: () => navigator.back()
+        ),
+        new RaisedButton(
+          key: 'b',
+          child: new Text('GO TO DUNGEON'),
+          onPressed: () => navigator.push(routes[2])
+        )
+      ])
     )
   ),
   new Route(
     name: 'adventure',
-    builder: (navigator) => new RaisedButton(
-      child: new Text('NO WAIT! GO BACK!'),
-      onPressed: () => navigator.pushRoute(routes[0])
+    builder: (navigator) => new Container(
+      padding: const EdgeDims.all(20.0),
+      decoration: new BoxDecoration(backgroundColor: const Color(0xFFDC143C)),
+      child: new Block([
+        new Text("Monster's Lair"),
+        new RaisedButton(
+          child: new Text('NO WAIT! GO BACK!'),
+          onPressed: () => navigator.pop()
+        )
+      ])
     )
   )
 ];
 
 class NavigationExampleApp extends App {
-  UINode build() {
-    return new Navigator(routes: routes);
+  Widget build() {
+    return new Flex([new Navigator(routes: routes)]);
   }
 }