add carousel, card_swipe, and focus_image samples (#119)

* add carousel, card_swipe, and focus_image samples

* fix image assets

* fix more asset images

* add repeating animation

* fix import

* add copyright headers

* remove Center widget

* imageAssetName

* use ClipRect, refactor _SwipeableCardState

* use offset.zero

* add comments

* remove reference to coverflow package

* change spread to toList()

* refactor coverflow sample

don't set width and height
use const
data -> images

* return widget directly, fix formatting

* inline transitionsBuilder

* image -> imageAssetName

* _rectTween() => _createTween()

* _expandToPageRoute -> _createRoute

* move non-updating widgets out of AnimatedBuilder

* code review updates to animations demos

animations/lib/src/misc/repeating_animation.dart

@@ -0,0 +1,60 @@
+// 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 RepeatingAnimationDemo extends StatefulWidget {
+  static String routeName = '/misc/repeating_animation';
+
+  @override
+  RepeatingAnimationDemoState createState() => RepeatingAnimationDemoState();
+}
+
+class RepeatingAnimationDemoState extends State<RepeatingAnimationDemo>
+    with SingleTickerProviderStateMixin {
+  AnimationController _controller;
+  Animation<BorderRadius> _borderRadius;
+
+  @override
+  void initState() {
+    super.initState();
+
+    _controller =
+        AnimationController(duration: const Duration(seconds: 2), vsync: this)
+          ..repeat(reverse: true);
+
+    _borderRadius = BorderRadiusTween(
+      begin: BorderRadius.circular(100.0),
+      end: BorderRadius.circular(0.0),
+    ).animate(_controller);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: Text('Repeating Animation')),
+      body: Center(
+        child: AnimatedBuilder(
+          animation: _borderRadius,
+          builder: (context, child) {
+            return Container(
+              width: 200,
+              height: 200,
+              decoration: BoxDecoration(
+                color: Colors.deepPurple,
+                borderRadius: _borderRadius.value,
+              ),
+            );
+          },
+        ),
+      ),
+    );
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+}