serve method
Starts this server at uri and will log a message returned from logMessage
when this server is successfully started.
This method must be called only once.
Implementation
Future<Null> serve(String logMessage(HttpServer server)) async {
if (_serveHasBeenCalled)
throw new StateError(
'For each instance, `serve` must be called only once.');
_serveHasBeenCalled = true;
_server = await io.serve(
pipeline.addHandler(proxyHandler(to)), uri.host, uri.port);
logger.info(logMessage(_server));
}