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/material/persistent_bottom_sheet_demo.dart

@@ -1,8 +1,8 @@
-// Copyright 2018 The Chromium Authors. All rights reserved.
+// Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter_web/material.dart';
+import 'package:flutter/material.dart';
 
 import '../../gallery/demo.dart';
 
@@ -10,8 +10,7 @@ class PersistentBottomSheetDemo extends StatefulWidget {
   static const String routeName = '/material/persistent-bottom-sheet';
 
   @override
-  _PersistentBottomSheetDemoState createState() =>
-      _PersistentBottomSheetDemoState();
+  _PersistentBottomSheetDemoState createState() => _PersistentBottomSheetDemoState();
 }
 
 class _PersistentBottomSheetDemoState extends State<PersistentBottomSheetDemo> {
@@ -26,36 +25,34 @@ class _PersistentBottomSheetDemoState extends State<PersistentBottomSheetDemo> {
   }
 
   void _showBottomSheet() {
-    setState(() {
-      // disable the button
+    setState(() { // disable the button
       _showBottomSheetCallback = null;
     });
-    _scaffoldKey.currentState
-        .showBottomSheet<Null>((BuildContext context) {
-          final ThemeData themeData = Theme.of(context);
-          return Container(
-            decoration: BoxDecoration(
-                border:
-                    Border(top: BorderSide(color: themeData.disabledColor))),
-            child: Padding(
-              padding: const EdgeInsets.all(32.0),
-              child: Text(
-                'This is a Material persistent bottom sheet. Drag downwards to dismiss it.',
-                textAlign: TextAlign.center,
-                style: TextStyle(color: themeData.accentColor, fontSize: 24.0),
-              ),
+    _scaffoldKey.currentState.showBottomSheet<void>((BuildContext context) {
+      final ThemeData themeData = Theme.of(context);
+      return Container(
+        decoration: BoxDecoration(
+          border: Border(top: BorderSide(color: themeData.disabledColor))
+        ),
+        child: Padding(
+          padding: const EdgeInsets.all(32.0),
+          child: Text('This is a Material persistent bottom sheet. Drag downwards to dismiss it.',
+            textAlign: TextAlign.center,
+            style: TextStyle(
+              color: themeData.accentColor,
+              fontSize: 24.0,
             ),
-          );
-        })
-        .closed
-        .whenComplete(() {
-          if (mounted) {
-            setState(() {
-              // re-enable the button
-              _showBottomSheetCallback = _showBottomSheet;
-            });
-          }
+          ),
+        ),
+      );
+    })
+    .closed.whenComplete(() {
+      if (mounted) {
+        setState(() { // re-enable the button
+          _showBottomSheetCallback = _showBottomSheet;
         });
+      }
+    });
   }
 
   void _showMessage() {
@@ -66,10 +63,11 @@ class _PersistentBottomSheetDemoState extends State<PersistentBottomSheetDemo> {
           content: const Text('You tapped the floating action button.'),
           actions: <Widget>[
             FlatButton(
-                onPressed: () {
-                  Navigator.pop(context);
-                },
-                child: const Text('OK'))
+              onPressed: () {
+                Navigator.pop(context);
+              },
+              child: const Text('OK'),
+            ),
           ],
         );
       },
@@ -79,25 +77,27 @@ class _PersistentBottomSheetDemoState extends State<PersistentBottomSheetDemo> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-        key: _scaffoldKey,
-        appBar: AppBar(
-          title: const Text('Persistent bottom sheet'),
-          actions: <Widget>[
-            MaterialDemoDocumentationButton(
-                PersistentBottomSheetDemo.routeName),
-          ],
+      key: _scaffoldKey,
+      appBar: AppBar(
+        title: const Text('Persistent bottom sheet'),
+        actions: <Widget>[
+          MaterialDemoDocumentationButton(PersistentBottomSheetDemo.routeName),
+        ],
+      ),
+      floatingActionButton: FloatingActionButton(
+        onPressed: _showMessage,
+        backgroundColor: Colors.redAccent,
+        child: const Icon(
+          Icons.add,
+          semanticLabel: 'Add',
         ),
-        floatingActionButton: FloatingActionButton(
-          onPressed: _showMessage,
-          backgroundColor: Colors.redAccent,
-          child: const Icon(
-            Icons.add,
-            semanticLabel: 'Add',
-          ),
+      ),
+      body: Center(
+        child: RaisedButton(
+          onPressed: _showBottomSheetCallback,
+          child: const Text('SHOW BOTTOM SHEET'),
         ),
-        body: Center(
-            child: RaisedButton(
-                onPressed: _showBottomSheetCallback,
-                child: const Text('SHOW BOTTOM SHEET'))));
+      ),
+    );
   }
 }