Support print on web

[hayhv]

When i run 'flutter build apk' it run success but if i run on web 'flutter run -d chrome' it do not connect to printer

[jocelyngirard]

This issue is related to Socket from dart.io, not compatible on Flutter Web.

Related to https://stackoverflow.com/questions/60266060/how-can-i-use-sockets-with-flutter-web : You can use HtmlWebSocketChannel from the web_socket_channel package as documented here. I have used this package and can confirm it works well in flutter web.

@andrey-ushakov, from your view, the socket used by POS is compatible with web socket implementation ?

[baotran310702]

If it connected, is it able to print on web??