Update web/ samples to work with Flutter SDK (#134)

* add package:http dependency in dad_jokes

* add package:http dependency in filipino_cuisine

* don't build package:http demos until flutter/flutter#34858 is resolved

* update gallery

* update github_dataviz

* update particle_background

* don't build github_dataviz (uses package:http)

* update slide_puzzle

* update spinning_square

* update timeflow

* update vision_challenge

* update charts

* update dad_jokes

* update filipino cuisine

* ignore build output

* update timeflow and vision_challenge

* update slide_puzzle

* don't commit build/ directory

* move preview.png images to assets

* fix path url join

* update readme

* update web/readme.md

web/charts/lib/scatter_plot_chart/scatter_plot_gallery.dart

@@ -0,0 +1,58 @@
+// 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/material.dart';
+import '../gallery_scaffold.dart';
+import 'animation_zoom.dart';
+import 'bucketing_axis.dart';
+import 'comparison_points.dart';
+import 'shapes.dart';
+import 'simple.dart';
+
+List<GalleryScaffold> buildGallery() {
+  return [
+    new GalleryScaffold(
+      listTileIcon: new Icon(Icons.scatter_plot),
+      title: 'Simple Scatter Plot Chart',
+      subtitle: 'With a single series',
+      childBuilder: () => new SimpleScatterPlotChart.withRandomData(),
+    ),
+    new GalleryScaffold(
+      listTileIcon: new Icon(Icons.scatter_plot),
+      title: 'Shapes Scatter Plot Chart',
+      subtitle: 'With custom shapes',
+      childBuilder: () => new ShapesScatterPlotChart.withRandomData(),
+    ),
+    new GalleryScaffold(
+      listTileIcon: new Icon(Icons.scatter_plot),
+      title: 'Comparison Points Scatter Plot Chart',
+      subtitle: 'Scatter plot chart with comparison points',
+      childBuilder: () => new ComparisonPointsScatterPlotChart.withRandomData(),
+    ),
+    new GalleryScaffold(
+      listTileIcon: new Icon(Icons.scatter_plot),
+      title: 'Pan and Zoom Scatter Plot Chart',
+      subtitle: 'Simple scatter plot chart pan and zoom behaviors enabled',
+      childBuilder: () => new ScatterPlotAnimationZoomChart.withRandomData(),
+    ),
+    new GalleryScaffold(
+      listTileIcon: new Icon(Icons.scatter_plot),
+      title: 'Bucketing Axis Scatter Plot Chart',
+      subtitle: 'Scatter plot with a measure axis that buckets values less ' +
+          'than 10% into a single region below the draw area',
+      childBuilder: () => new BucketingAxisScatterPlotChart.withRandomData(),
+    ),
+  ];
+}