Tidyup animations sample (#1672)

* Tidyup

* Drop `flutterAnimateDemos`

animations/lib/src/basics/animation_controller.dart

@@ -0,0 +1,87 @@
+// 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 {
+  const AnimationControllerDemo({super.key});
+  static const String routeName = 'basics/animation_controller';
+
+  @override
+  State<AnimationControllerDemo> createState() =>
+      _AnimationControllerDemoState();
+}
+
+class _AnimationControllerDemoState extends State<AnimationControllerDemo>
+    with SingleTickerProviderStateMixin {
+  // Using the SingleTickerProviderStateMixin can ensure that our
+  // AnimationController only animates while the Widget is visible on the
+  // screen. This is a useful optimization that saves resources when the
+  // Widget is not visible.
+
+  static const Duration _duration = Duration(seconds: 1);
+  late final AnimationController controller;
+
+  @override
+  void initState() {
+    super.initState();
+
+    controller = AnimationController(vsync: this, duration: _duration)
+      // The Widget's build needs to be called every time the animation's
+      // value changes. So add a listener here that will call setState()
+      // and trigger the build() method to be called by the framework.
+      // If your Widget's build is relatively simple, this is a good option.
+      // However, if your build method returns a tree of child Widgets and
+      // most of them are not animated you should consider using
+      // AnimatedBuilder instead.
+      ..addListener(() {
+        setState(() {});
+      });
+  }
+
+  @override
+  void dispose() {
+    // AnimationController is a stateful resource that needs to be disposed when
+    // this State gets disposed.
+    controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    // When building the widget you can read the AnimationController's value property
+    // when building child widgets. You can also check the status to see if the animation
+    // has completed.
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text('Animation Controller'),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            ConstrainedBox(
+              constraints: const BoxConstraints(maxWidth: 200),
+              child: Text(
+                controller.value.toStringAsFixed(2),
+                style: Theme.of(context).textTheme.displaySmall,
+                textScaleFactor: 1 + controller.value,
+              ),
+            ),
+            ElevatedButton(
+              child: const Text('animate'),
+              onPressed: () {
+                if (controller.status == AnimationStatus.completed) {
+                  controller.reverse();
+                } else {
+                  controller.forward();
+                }
+              },
+            )
+          ],
+        ),
+      ),
+    );
+  }
+}