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/images_demo.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+
+import '../gallery/demo.dart';
+
+class ImagesDemo extends StatelessWidget {
+  static const String routeName = '/images';
+
+  @override
+  Widget build(BuildContext context) {
+    return TabbedComponentDemoScaffold(
+      title: 'Animated images',
+      demos: <ComponentDemoTabData>[
+        ComponentDemoTabData(
+          tabName: 'WEBP',
+          description: '',
+          exampleCodeTag: 'animated_image',
+          demoWidget: Semantics(
+            label: 'Example of animated WEBP',
+            child: Image.asset(
+              'animated_images/animated_flutter_stickers.webp',
+              package: 'flutter_gallery_assets',
+            ),
+          ),
+        ),
+        ComponentDemoTabData(
+          tabName: 'GIF',
+          description: '',
+          exampleCodeTag: 'animated_image',
+          demoWidget: Semantics(
+            label: 'Example of animated GIF',
+            child:Image.asset(
+              'animated_images/animated_flutter_lgtm.gif',
+              package: 'flutter_gallery_assets',
+            ),
+          ),
+        ),
+      ],
+    );
+  }
+}