Add flutter_web samples (#75)

web/charts/flutter/lib/src/util.dart

@@ -0,0 +1,45 @@
+// Copyright 2018 the Charts project authors. Please see the AUTHORS file
+// for details.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import 'package:flutter_web/rendering.dart'
+    show
+        RenderBox,
+        RenderSemanticsGestureHandler,
+        RenderPointerListener,
+        RenderCustomMultiChildLayoutBox;
+import 'chart_container.dart' show ChartContainerRenderObject;
+
+/// Get the [ChartContainerRenderObject] from a [RenderBox].
+///
+/// [RenderBox] is expected to be a [RenderSemanticsGestureHandler] with child
+/// of [RenderPointerListener] with child of [ChartContainerRenderObject].
+ChartContainerRenderObject getChartContainerRenderObject(RenderBox box) {
+  assert(box is RenderCustomMultiChildLayoutBox);
+  final semanticHandler = (box as RenderCustomMultiChildLayoutBox)
+      .getChildrenAsList()
+      .firstWhere((child) => child is RenderSemanticsGestureHandler);
+
+  assert(semanticHandler is RenderSemanticsGestureHandler);
+  final renderPointerListener =
+      (semanticHandler as RenderSemanticsGestureHandler).child;
+
+  assert(renderPointerListener is RenderPointerListener);
+  final chartContainerRenderObject =
+      (renderPointerListener as RenderPointerListener).child;
+
+  assert(chartContainerRenderObject is ChartContainerRenderObject);
+
+  return chartContainerRenderObject as ChartContainerRenderObject;
+}