Dart integration test with VM server and dartium browser

I'm making a library that implements both server and client parts that interacts between them via websockets:

Server use example (ran in CLI):

Server srv = new Server("localhost:1234");
srv.onNewClientConnected.listen(print("client connected"));

Client use example (ran in browser):

Client cli = new Cliente("localhost:1234");
cli.sendCommand(...);

Just by creating the instances, the client should be connected and the server noticed about that connection. What would be the best way to test their interactions? Could I check both objects internals?