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/material/elevation_demo.dart

@@ -1,8 +1,4 @@
-// Copyright 2018 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_web/material.dart';
+import 'package:flutter/material.dart';
 
 import '../../gallery/demo.dart';
 
@@ -58,12 +54,10 @@ class _ElevationDemoState extends State<ElevationDemo> {
             onPressed: () {
               setState(() => _showElevation = !_showElevation);
             },
-          )
+          ),
         ],
       ),
-      body: ListView(
-        children: buildCards(),
-      ),
+      body: Scrollbar(child: ListView(children: buildCards())),
     );
   }
 }