Unsupported operation: Platform._operatingSystem

I reopen this issue to give a more appropriate answer which is now available in native in flutter:

import 'package:flutter/foundation.dart';

if (kIsWeb) {
    // Some web specific code there
}
else if (defaultTargetPlatform == TargetPlatform.iOS || defaultTargetPlatform == TargetPlatform.android) {
    // Some android/ios specific code
}
else if (defaultTargetPlatform == TargetPlatform.linux || defaultTargetPlatform == TargetPlatform.macOS || defaultTargetPlatform == TargetPlatform.windows) {
    // Some desktop specific code there
}