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/src/basics/animation_controller_demo.dart

@@ -0,0 +1,67 @@
+// 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';
+
+class AnimationControllerDemo extends StatefulWidget {
+  static const String routeName = '/basics/animation_controller';
+
+  @override
+  _AnimationControllerDemoState createState() =>
+      _AnimationControllerDemoState();
+}
+
+class _AnimationControllerDemoState extends State<AnimationControllerDemo>
+    with SingleTickerProviderStateMixin {
+  static const Duration _duration = Duration(seconds: 1);
+  AnimationController controller;
+
+  @override
+  void initState() {
+    super.initState();
+
+    controller = AnimationController(vsync: this, duration: _duration)
+      ..addListener(() {
+        // Force build() to be called again
+        setState(() {});
+      });
+  }
+
+  @override
+  void dispose() {
+    super.dispose();
+    controller.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            ConstrainedBox(
+              constraints: BoxConstraints(maxWidth: 200),
+              child: Text(
+                '${controller.value.toStringAsFixed(2)}',
+                style: Theme.of(context).textTheme.display3,
+              ),
+            ),
+            RaisedButton(
+              child: Text('animate'),
+              onPressed: () {
+                if (controller.status == AnimationStatus.completed) {
+                  controller.reverse();
+                } else {
+                  controller.forward();
+                }
+              },
+            )
+          ],
+        ),
+      ),
+    );
+  }
+}