Add scroll physics demo (#123)

* add spring physics card demo

* fix button colors in animations project

* code review updates

* refactor method into runAnimation

* combine updateAnimation and runAnimation

* update comment

animations/lib/src/basics/02_page_route_builder.dart

@@ -9,7 +9,6 @@ class PageRouteBuilderDemo extends StatelessWidget {
       body: Center(
         child: RaisedButton(
           child: Text('Go!'),
-          color: Theme.of(context).primaryColor,
           onPressed: () {
             Navigator.of(context).push(_createRoute());
           },

animations/lib/src/basics/05_custom_tween.dart

@@ -43,12 +43,13 @@ class _CustomTweenDemoState extends State<CustomTweenDemo>
     return Scaffold(
       appBar: AppBar(
         actions: [
-          RaisedButton(
+          MaterialButton(
             child: Text(
               controller.status == AnimationStatus.completed
                   ? 'Delete Essay'
                   : 'Write Essay',
             ),
+            textColor: Colors.white,
             onPressed: () {
               if (controller.status == AnimationStatus.completed) {
                 controller.reverse();