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/pubspec.yaml

@@ -4,20 +4,10 @@ environment:
   sdk: ">=2.2.0 <3.0.0"
 
 dependencies:
-  flutter_web: any
-
-dev_dependencies:
-  build_runner: any
-  build_web_compilers: any
-
-# flutter_web packages are not published to pub.dartlang.org
-# These overrides tell the package tools to get them from GitHub
-dependency_overrides:
-  flutter_web:
-    git:
-      url: https://github.com/flutter/flutter_web
-      path: packages/flutter_web
-  flutter_web_ui:
-    git:
-      url: https://github.com/flutter/flutter_web
-      path: packages/flutter_web_ui
+  flutter:
+    sdk: flutter
+  intl: ^0.16.0
+  http: ^0.12.0
+flutter:
+  assets:
+    - preview.png