Add flutter_web samples (#75)

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

@@ -0,0 +1,36 @@
+// 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.
+
+abstract class ChartState {
+  void setAnimation(Duration transition);
+
+  /// Request to the native platform to rebuild the chart.
+  void requestRebuild();
+
+  /// Informs the chart that the configuration has changed.
+  ///
+  /// This flag is set by checks that detect if a configuration has changed,
+  /// such as behaviors, axis, and renderers.
+  ///
+  /// This flag is read on chart rebuild, if chart is marked as dirty, then the
+  /// chart will call a base chart draw.
+  void markChartDirty();
+
+  /// Reset the chart dirty flag.
+  void resetChartDirtyFlag();
+
+  /// Gets if the chart is dirty.
+  bool get chartIsDirty;
+}