FakeTransport constructor
Creates a fake transport.
With autoReady false, ready stays pending until completeReady or
failReady is called, which is how a slow or refused connection is
tested. onSend sees every frame as it is sent, which is how
MockSocketServer answers them.
Implementation
FakeTransport({bool autoReady = true, this.onSend}) {
if (autoReady) _ready.complete();
}