The built-in library 'dart:io' is not available on Dartium

I don't know where is the problem. This error show up when I import my SystemCheck class to main Dart file.

SystemCheck class:

import 'dart:io';
class SystemCheck{  
    getOperatingSystem() => Platform.operatingSystem;
    getUser() => Platform.localHostname;  
}

Import in main file:

import 'cz.felguide.core/system.dart';