Moved two animation vignettes into misc. (#425)

animations/lib/src/misc/animated_positioned.dart

@@ -0,0 +1,79 @@
+// Copyright 2020 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 'dart:math';
+
+import 'package:flutter/material.dart';
+
+class AnimatedPositionedDemo extends StatefulWidget {
+  static String routeName = '/basics/09_animated_positioned';
+
+  _AnimatedPositionedDemoState createState() => _AnimatedPositionedDemoState();
+}
+
+class _AnimatedPositionedDemoState extends State<AnimatedPositionedDemo> {
+  double topPosition;
+  double leftPosition;
+
+  double generateTopPosition(double top) => Random().nextDouble() * top;
+
+  double generateLeftPosition(double left) => Random().nextDouble() * left;
+
+  void initState() {
+    super.initState();
+    topPosition = generateTopPosition(30);
+    leftPosition = generateLeftPosition(30);
+  }
+
+  void changePosition(double top, double left) {
+    setState(() {
+      topPosition = generateTopPosition(top);
+      leftPosition = generateLeftPosition(left);
+    });
+  }
+
+  Widget build(BuildContext context) {
+    final size = MediaQuery.of(context).size;
+    final appBar = AppBar();
+    final topPadding = MediaQuery.of(context).padding.top;
+    // AnimatedPositioned animates changes to a widget's position within a Stack
+    return SafeArea(
+      child: Scaffold(
+        appBar: appBar,
+        body: Container(
+          height: size.height,
+          width: size.width,
+          child: Stack(
+            children: [
+              AnimatedPositioned(
+                top: topPosition,
+                left: leftPosition,
+                duration: Duration(seconds: 1),
+                child: InkWell(
+                  onTap: () => changePosition(
+                      size.height -
+                          (appBar.preferredSize.height + topPadding + 50),
+                      size.width - 150),
+                  child: Container(
+                    alignment: Alignment.center,
+                    width: 150,
+                    height: 50,
+                    child: Text(
+                      "Click Me",
+                      style: TextStyle(
+                        color:
+                            Theme.of(context).buttonTheme.colorScheme.onPrimary,
+                      ),
+                    ),
+                    color: Theme.of(context).primaryColor,
+                  ),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}