add TweenSequence sample (#127)

* Add TweenSequence sample

* rename to TweenSequenceDemo

* add copyright headers

* remote type

* introduce animated builder earlier in basics/

so that other examples can use it

* formatting

* use async / await, increase animation duration

animations/lib/src/basics/01_animated_container.dart

@@ -1,3 +1,7 @@
+// 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 'dart:math';
 
 import 'package:flutter/material.dart';