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/_tool/peanut_post_build.dart

@@ -110,7 +110,7 @@ class _Demo {
   String get html => '''
 <div>
   <a href='$buildDir'>
-    <img src='${p.url.join(buildDir, 'preview.png')}' width="300" alt="$name">
+    <img src='${p.url.join(buildDir, 'assets/assets/preview.png')}' width="300" alt="$name">
   </a>
   <a class='demo-title' href='$buildDir'>$name</a>
   <div>