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/charts/pubspec.yaml

@@ -0,0 +1,13 @@
+name: example
+description: Charts-Flutter Demo
+dependencies:
+  flutter:
+    sdk: flutter
+  charts_flutter: any
+  meta: ^1.1.1
+  intl: ^0.15.2
+
+flutter:
+  uses-material-design: true
+  assets:
+    - preview.png