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/readme.md

@@ -1,22 +1,28 @@
-Samples for [Flutter for web](https://flutter.dev/web).
+Web samples
 
 ## See the samples in action
 
-Compiled versions of the samples are hosted at [flutter.github.io/samples](https://flutter.github.io/samples/).
+Compiled versions of the samples are hosted at
+[flutter.github.io/samples][samples].
 
 ## Building samples code
 
-Make sure you have configured the `webdev` tool.
-
-- [With the Flutter SDK installed](https://github.com/flutter/flutter_web#install-the-flutter_web-build-tools).
-- [With the Dart SDK installed](https://dart.dev/tools/webdev#using-webdev-and-build_runner-commands).
-
-Go into one of the sample directories, get packages, and run.
+Go into one of the sample directories, get packages, and run using the `chrome`
+device:
 
 ```console
+$ flutter channel dev
+$ flutter upgrade
 $ cd gallery
-$ flutter pub get # or `pub get` with the Dart SDK
-$ webdev serve
+$ flutter pub get
+$ flutter run -d chrome
 ```
 
+Web support is available as a technical preview and is only available in the
+`dev` or `master` channels.
+
 You should see a message printing the URL to access: `http://localhost:8080`
+
+[web]: https://flutter.dev/web
+[samples]: https://flutter.github.io/samples/
+