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/gallery/example_code_parser.dart

@@ -0,0 +1,55 @@
+// Copyright 2016 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 'dart:async';
+
+import 'package:flutter/services.dart';
+
+const String _kStartTag = '// START ';
+const String _kEndTag = '// END';
+
+Map<String, String> _exampleCode;
+
+Future<String> getExampleCode(String tag, AssetBundle bundle) async {
+  if (_exampleCode == null)
+    await _parseExampleCode(bundle);
+  return _exampleCode[tag];
+}
+
+Future<void> _parseExampleCode(AssetBundle bundle) async {
+  final String code = await bundle.loadString('lib/gallery/example_code.dart') ??
+    '// lib/gallery/example_code.dart not found\n';
+  _exampleCode = <String, String>{};
+
+  final List<String> lines = code.split('\n');
+
+  List<String> codeBlock;
+  String codeTag;
+
+  for (String line in lines) {
+    if (codeBlock == null) {
+      // Outside a block.
+      if (line.startsWith(_kStartTag)) {
+        // Starting a new code block.
+        codeBlock = <String>[];
+        codeTag = line.substring(_kStartTag.length).trim();
+      } else {
+        // Just skipping the line.
+      }
+    } else {
+      // Inside a block.
+      if (line.startsWith(_kEndTag)) {
+        // Add the block.
+        _exampleCode[codeTag] = codeBlock.join('\n');
+        codeBlock = null;
+        codeTag = null;
+      } else {
+        // Add to the current block
+        // trimRight() to remove any \r on Windows
+        // without removing any useful indentation
+        codeBlock.add(line.trimRight());
+      }
+    }
+  }
+}