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/fortnightly/README.md

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+# Fortnightly
+
+A Flutter sample app based on the Material study Fortnightly (a hypothetical, online newspaper.) It
+showcases print-quality, custom typography, Material Theming, and text-heavy UI design and layout.
+
+For info on the Fortnightly Material Study, see: https://material.io/design/material-studies/fortnightly.html
+
+## Goals for this sample
+
+* Help you understand how to customize and layout text.
+* Provide you with example code for
+  * Text
+  * A short app bar (the menu button top left.)
+  * Avatar images
+
+## Widgets / APIs
+
+* BeveledRectangleBorder
+* BoxConstraints on Container
+* CircleAvatar
+* ExactAssetImage
+* Fonts
+* SafeArea
+* Stack
+* SingleChildScrollView
+* Text
+* TextStyle
+* TextTheme
+
+## Notice
+
+* Theming is passed as a parameter in the constructor of `MaterialApp` (`theme:`).
+* `SafeArea` adds padding around notches and virtual home buttons on screens that have them (like
+    iPhone X+). Here, it protects the `ShortAppBar` from overlapping with the status bar (time)
+    and makes sure the bottom of the newspaper article has padding beneath it if necessary.
+* The entire newspaper article is wrapped in a `SingleChildScrollView` widget which ensures that the
+    entire article can be viewed no matter what the screen's size or orientation is.
+* The `Text` widget with text ' ¬ ' has a `TextStyle` that changes one parameter of an inherited
+    `TextStyle` using `.apply()``.
+* The `Text` widget with text 'Connor Eghan' has a `TextStyle` created explicitly instead of
+    inheriting from theming.
+* You can break up long strings in your source files by putting them on multiple lines.
+* Fonts are imported with multiple files expressing their weights (Bold, Light, Medium, Regular)
+    but are accessed with a `FontWeight` value like `FontWeight.w800` for Merriweather-Bold.ttf.
+
+## Questions/issues
+
+If you have a general question about developing in Flutter, the best places to go are:
+
+* [The FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
+* [The Flutter Gitter channel](https://gitter.im/flutter/flutter)
+* [StackOverflow](https://stackoverflow.com/questions/tagged/flutter)