Add flutter_web samples (#75)

web/slide_puzzle/lib/main.dart

@@ -0,0 +1,33 @@
+// Copyright 2019 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 'src/core/puzzle_animator.dart';
+import 'src/flutter.dart';
+import 'src/puzzle_home_state.dart';
+
+void main() => runApp(PuzzleApp());
+
+class PuzzleApp extends StatelessWidget {
+  final int rows, columns;
+
+  PuzzleApp({int columns = 4, int rows = 4})
+      : columns = columns ?? 4,
+        rows = rows ?? 4;
+
+  @override
+  Widget build(BuildContext context) => MaterialApp(
+        title: 'Slide Puzzle',
+        home: _PuzzleHome(rows, columns),
+      );
+}
+
+class _PuzzleHome extends StatefulWidget {
+  final int _rows, _columns;
+
+  const _PuzzleHome(this._rows, this._columns, {Key key}) : super(key: key);
+
+  @override
+  PuzzleHomeState createState() =>
+      PuzzleHomeState(PuzzleAnimator(_columns, _rows));
+}