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/gallery/lib/demo/cupertino/cupertino_slider_demo.dart

@@ -0,0 +1,78 @@
+// Copyright 2017 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 'package:flutter/cupertino.dart';
+
+import '../../gallery/demo.dart';
+
+class CupertinoSliderDemo extends StatefulWidget {
+  static const String routeName = '/cupertino/slider';
+
+  @override
+  _CupertinoSliderDemoState createState() => _CupertinoSliderDemoState();
+}
+
+class _CupertinoSliderDemoState extends State<CupertinoSliderDemo> {
+  double _value = 25.0;
+  double _discreteValue = 20.0;
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(
+        middle: const Text('Sliders'),
+        // We're specifying a back label here because the previous page is a
+        // Material page. CupertinoPageRoutes could auto-populate these back
+        // labels.
+        previousPageTitle: 'Cupertino',
+        trailing: CupertinoDemoDocumentationButton(CupertinoSliderDemo.routeName),
+      ),
+      child: DefaultTextStyle(
+        style: CupertinoTheme.of(context).textTheme.textStyle,
+        child: SafeArea(
+          child: Center(
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.spaceAround,
+              children: <Widget>[
+                Column(
+                  mainAxisSize: MainAxisSize.min,
+                  children: <Widget> [
+                    CupertinoSlider(
+                      value: _value,
+                      min: 0.0,
+                      max: 100.0,
+                      onChanged: (double value) {
+                        setState(() {
+                          _value = value;
+                        });
+                      },
+                    ),
+                    Text('Cupertino Continuous: ${_value.toStringAsFixed(1)}'),
+                  ],
+                ),
+                Column(
+                  mainAxisSize: MainAxisSize.min,
+                  children: <Widget> [
+                    CupertinoSlider(
+                      value: _discreteValue,
+                      min: 0.0,
+                      max: 100.0,
+                      divisions: 5,
+                      onChanged: (double value) {
+                        setState(() {
+                          _discreteValue = value;
+                        });
+                      },
+                    ),
+                    Text('Cupertino Discrete: $_discreteValue'),
+                  ],
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}