Add animation samples (#118)

* initialize animations sample project

* add two demos, set up routing

* clean up expand_card demo

* update README

* clean up expand_card demo

* dartfmt

* update comment

* address code review comments

* swap AnimatedContainer and AnimatedCrossFade

* use AnimatedSwitcher instead of AnimatedCrossFade

* Revert "use AnimatedSwitcher instead of AnimatedCrossFade"

This reverts commit e112e02549.

* rename expanded -> selected

* use  Dart 2.4.0 constraint

* update README

* address code review comments

update images
add curves to expand_card
update pubspec.lock

* add @override annotation

* add animations project to travis script

* add empty test for travis

* add copyright notice to animations/ project

animations/lib/main.dart

@@ -0,0 +1,85 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'src/basics/animation_controller_demo.dart';
+import 'src/misc/expand_card.dart';
+
+void main() => runApp(AnimationSamples());
+
+class Demo {
+  final String name;
+  final String route;
+  final WidgetBuilder builder;
+
+  const Demo(this.name, this.route, this.builder);
+}
+
+final basicDemos = [
+  Demo('Animation Controller', AnimationControllerDemo.routeName,
+      (context) => AnimationControllerDemo()),
+];
+
+final miscDemos = [
+  Demo('Expandable Card', ExpandCardDemo.routeName,
+      (context) => ExpandCardDemo()),
+];
+
+final basicDemoRoutes =
+    Map.fromEntries(basicDemos.map((d) => MapEntry(d.route, d.builder)));
+
+final miscDemoRoutes =
+    Map.fromEntries(miscDemos.map((d) => MapEntry(d.route, d.builder)));
+
+final allRoutes = <String, WidgetBuilder>{
+  ...basicDemoRoutes,
+  ...miscDemoRoutes,
+};
+
+class AnimationSamples extends StatelessWidget {
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Animation Samples',
+      theme: ThemeData(
+        primarySwatch: Colors.deepPurple,
+      ),
+      routes: allRoutes,
+      home: HomePage(),
+    );
+  }
+}
+
+class HomePage extends StatelessWidget {
+  Widget build(BuildContext context) {
+    final headerStyle = Theme.of(context).textTheme.title;
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Animation Samples'),
+      ),
+      body: ListView(
+        children: [
+          ListTile(title: Text('Basics', style: headerStyle)),
+          ...basicDemos.map((d) => DemoTile(d)),
+          ListTile(title: Text('Misc', style: headerStyle)),
+          ...miscDemos.map((d) => DemoTile(d)),
+        ],
+      ),
+    );
+  }
+}
+
+class DemoTile extends StatelessWidget {
+  final Demo demo;
+
+  DemoTile(this.demo);
+
+  Widget build(BuildContext context) {
+    return ListTile(
+      title: Text(demo.name),
+      onTap: () {
+        Navigator.pushNamed(context, demo.route);
+      },
+    );
+  }
+}