Added support for WebSockets

[Martoxdlol]

Hello!
I added support for proxying websockets. It is implemented checking the 'Upgrade' header and using shelf_web_socket and web_socket_channel packages.

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[Martoxdlol]

Hello, could someone review this?

[kevmoo]

@natebosch @devoncarew @brianquinlan – thoughts?

[natebosch]

This package currently does not have imports to dart:io. I don't know that there is utility in running this code anywhere without dart:io, but adding the new dependencies and the new platform restriction is probably enough reason to not add this support.

I wonder if there is some way can make this pluggable.

Or is the import 'package:web_socket_channel/io.dart'; required - maybe we can use the cross platform import for connecting the socket?

[Martoxdlol]

This package currently does not have imports to dart:io. I don't know that there is utility in running this code anywhere without dart:io, but adding the new dependencies and the new platform restriction is probably enough reason to not add this support.

I wonder if there is some way can make this pluggable.

Or is the import 'package:web_socket_channel/io.dart'; required - maybe we can use the cross platform import for connecting the socket?

Hmmm I didn't think about that. I will look for another solution.

Maybe I can make a separate package that adds websockets or something.