Add:创建 flutter go web 版

lib/widgets/elements/Form/Button/PopupMenuButton/index.dart

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+/// Created with Android Studio.
+/// User: 一晟
+/// Date: 2018/11/24
+/// Time: 下午5:25
+/// email: zhu.yan@alibaba-inc.com
+/// target: PopupMenusButton 的示例
+/// 对应文档地址:https://docs.flutter.io/flutter/material/PopupMenuButton-class.html
+
+import 'package:flutter_web/material.dart';
+
+import 'package:flutter_go/components/widget_demo.dart';
+import './demo.dart' as popupMenuButton;
+
+const String _titleText0 = """
+### **简介**
+> 按下时显示菜单
+- 当菜单因为选择了项目而被解除时调用onSelected。传递给onSelected的值是所选菜单项的值;
+- 可以提供 `child` 或 `icon` 中的一个，但不是能同时设置两者。如果提供了 `icon` ，则 `PopupMenuButton` 的行为类似于 `IconButton`;
+- 如果两者都为null，则创建一个标准 overflow icon（取决于平台）;
+""";
+const String _titleText1 = """
+### **基本用法**
+> 此示例显示一个包含四个项目的菜单
+- 在枚举值之间进行选择，并_selection根据选择设置字段;
+""";
+
+const String _titleText2 = """
+### **进阶用法**
+> 此示例尝试调整所有属性，展示出效果
+- 默认选择第二个;
+- 再次打开,`PopupMenuItem` 保留上次的选择结果;
+""";
+
+class Demo extends StatefulWidget {
+  static const String routeName = '/element/Form/Button/PopupMenuButton';
+  final Map<String,String> options = {'defaultSelect': '选项二的内容'};
+  final _DemoState self =  _DemoState();
+
+  @override
+  _DemoState createState() => self;
+}
+
+class _DemoState extends State<Demo> {
+  String buttonShapeType = 'border'; // 边框类型
+  void setButtonShapeType(){
+    String _buttonShapeType = (buttonShapeType == 'border') ? 'radius' : 'border';
+    this.setState((){
+      buttonShapeType = _buttonShapeType;
+    });
+  }
+  @override
+  Widget build(BuildContext context) {
+    return WidgetDemo(
+      title: 'PopupMenuButton',
+      codeUrl: 'elements/Form/Button/RaisedButton/demo.dart',
+      contentList: allPopupMenuButton(widget,this),
+      docUrl: 'https://docs.flutter.io/flutter/material/PopupMenuButton-class.html',
+    );
+  }
+}
+
+List allPopupMenuButton(Demo widget, State parent) {
+  return [
+    _titleText0,
+    SizedBox(height: 20.0),
+    _titleText1,
+    Row(
+      crossAxisAlignment: CrossAxisAlignment.center,
+      mainAxisAlignment: MainAxisAlignment.spaceBetween,
+      children: <Widget>[
+        popupMenuButton.PopupMenuButtonDefault('default1'),
+        popupMenuButton.PopupMenuButtonDefault('default2'),
+        popupMenuButton.PopupMenuButtonDefault('default3'),
+      ],
+    ),
+    SizedBox(height: 20.0),
+    _titleText2,
+    SizedBox(height: 20.0),
+    popupMenuButton.PopupMenuButtonCustom(widget, parent),
+    SizedBox(height: 40.0)
+  ];
+}