Tidyup animations sample (#1672)

* Tidyup

* Drop `flutterAnimateDemos`

animations/lib/src/basics/fade_transition.dart

@@ -0,0 +1,77 @@
+// 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';
+
+// Refer to the AnimatedWidget docs here - https://api.flutter.dev/flutter/widgets/AnimatedWidget-class.html
+// for examples of other common animated widgets.
+class FadeTransitionDemo extends StatefulWidget {
+  const FadeTransitionDemo({super.key});
+  static const String routeName = 'basics/fade_transition';
+
+  @override
+  State<FadeTransitionDemo> createState() => _FadeTransitionDemoState();
+}
+
+class _FadeTransitionDemoState extends State<FadeTransitionDemo>
+    with SingleTickerProviderStateMixin {
+  late final AnimationController _controller;
+  late final Animation<double> _animation;
+  late final CurvedAnimation _curve;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = AnimationController(
+      vsync: this,
+      duration: const Duration(milliseconds: 500),
+    );
+
+    _curve = CurvedAnimation(parent: _controller, curve: Curves.easeIn);
+
+    _animation = Tween(
+      begin: 1.0,
+      end: 0.0,
+    ).animate(_curve);
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text(
+          'Fade Transition',
+        ),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            FadeTransition(
+              opacity: _animation,
+              child: const Icon(
+                Icons.star,
+                color: Colors.amber,
+                size: 300,
+              ),
+            ),
+            ElevatedButton(
+              child: const Text('animate'),
+              onPressed: () => setState(() {
+                _controller.animateTo(1.0).then<TickerFuture>(
+                    (value) => _controller.animateBack(0.0));
+              }),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}