Add flutter_web samples (#75)

web/slide_puzzle/lib/src/puzzle_flow_delegate.dart

@@ -0,0 +1,43 @@
+// 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 'core/puzzle_animator.dart';
+import 'flutter.dart';
+
+class PuzzleFlowDelegate extends FlowDelegate {
+  final Size _tileSize;
+  final PuzzleProxy _puzzleProxy;
+
+  PuzzleFlowDelegate(this._tileSize, this._puzzleProxy, Listenable repaint)
+      : super(repaint: repaint);
+
+  @override
+  Size getSize(BoxConstraints constraints) => Size(
+      _tileSize.width * _puzzleProxy.width,
+      _tileSize.height * _puzzleProxy.height);
+
+  @override
+  BoxConstraints getConstraintsForChild(int i, BoxConstraints constraints) =>
+      BoxConstraints.tight(_tileSize);
+
+  @override
+  void paintChildren(FlowPaintingContext context) {
+    for (var i = 0; i < _puzzleProxy.length; i++) {
+      final tileLocation = _puzzleProxy.location(i);
+      context.paintChild(
+        i,
+        transform: Matrix4.translationValues(
+          tileLocation.x * _tileSize.width,
+          tileLocation.y * _tileSize.height,
+          i.toDouble(),
+        ),
+      );
+    }
+  }
+
+  @override
+  bool shouldRepaint(covariant PuzzleFlowDelegate oldDelegate) =>
+      _tileSize != oldDelegate._tileSize ||
+      !identical(oldDelegate._puzzleProxy, _puzzleProxy);
+}