Add flutter_web samples (#75)

web/spinning_square/lib/main.dart

@@ -0,0 +1,48 @@
+// Copyright 2018 The Chromium Authors. 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_web/material.dart';
+
+class SpinningSquare extends StatefulWidget {
+  @override
+  _SpinningSquareState createState() => new _SpinningSquareState();
+}
+
+class _SpinningSquareState extends State<SpinningSquare>
+    with SingleTickerProviderStateMixin {
+  AnimationController _animation;
+
+  @override
+  void initState() {
+    super.initState();
+    // We use 3600 milliseconds instead of 1800 milliseconds because 0.0 -> 1.0
+    // represents an entire turn of the square whereas in the other examples
+    // we used 0.0 -> math.pi, which is only half a turn.
+    _animation = new AnimationController(
+      duration: const Duration(milliseconds: 3600),
+      vsync: this,
+    )..repeat();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new RotationTransition(
+        turns: _animation,
+        child: new Container(
+          width: 200.0,
+          height: 200.0,
+          color: const Color(0xFF00FF00),
+        ));
+  }
+
+  @override
+  void dispose() {
+    _animation.dispose();
+    super.dispose();
+  }
+}
+
+void main() {
+  runApp(new Center(child: new SpinningSquare()));
+}