提交：修改AppBar底部蒙蒙的一横条阴影；

lib/widget/my_app_bar.dart

+import 'dart:io';
+
 import 'package:flutter/material.dart';
 
 class MyPageAppBar extends StatelessWidget implements PreferredSizeWidget {
@@ -23,7 +25,7 @@ class MyPageAppBar extends StatelessWidget implements PreferredSizeWidget {
     this.actions,
     this.exitApp = false,
     this.leading,
-    this.elevation,
+    this.elevation = 0,
   }) : super(key: key);
 
   @override