Tidyup animations sample (#1672)

* Tidyup

* Drop `flutterAnimateDemos`

animations/lib/src/basics/tweens.dart

@@ -0,0 +1,74 @@
+// 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 TweenDemo extends StatefulWidget {
+  const TweenDemo({super.key});
+  static const String routeName = 'basics/tweens';
+
+  @override
+  State<TweenDemo> createState() => _TweenDemoState();
+}
+
+class _TweenDemoState extends State<TweenDemo>
+    with SingleTickerProviderStateMixin {
+  static const Duration _duration = Duration(seconds: 1);
+  static const double accountBalance = 1000000;
+  late final AnimationController controller;
+  late final Animation<double> animation;
+
+  @override
+  void initState() {
+    super.initState();
+
+    controller = AnimationController(vsync: this, duration: _duration)
+      ..addListener(() {
+        // Marks the widget tree as dirty
+        setState(() {});
+      });
+    animation = Tween(begin: 0.0, end: accountBalance).animate(controller);
+  }
+
+  @override
+  void dispose() {
+    controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text('Tweens'),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            ConstrainedBox(
+              constraints: const BoxConstraints(maxWidth: 200),
+              child: Text('\$${animation.value.toStringAsFixed(2)}',
+                  style: const TextStyle(fontSize: 24)),
+            ),
+            ElevatedButton(
+              child: Text(
+                controller.status == AnimationStatus.completed
+                    ? 'Buy a Mansion'
+                    : 'Win Lottery',
+              ),
+              onPressed: () {
+                if (controller.status == AnimationStatus.completed) {
+                  controller.reverse();
+                } else {
+                  controller.forward();
+                }
+              },
+            )
+          ],
+        ),
+      ),
+    );
+  }
+}