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/github_dataviz/web/main.dart

@@ -1,10 +0,0 @@
-// Copyright 2019 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_ui/ui.dart' as ui;
-import 'package:github_dataviz/main.dart' as app;
-
-main() async {
-  await ui.webOnlyInitializePlatform();
-  app.main();
-}