我本人只是遇到了这个问题，看起来dart:io库不适用于Web应用程序.

I just ran into this problem myself, looks like the dart:io library won't work with web applications.

来自: https://api.flutter.dev/flutter/dart-io/dart-io-library.html

重要提示:基于浏览器的应用程序无法使用此库.只有服务器，命令行脚本和Flutter移动应用程序才能导入和使用dart:io.

Important: Browser-based applications can't use this library. Only servers, command-line scripts, and Flutter mobile apps can import and use dart:io.