agreed that dont use devmode for server code is the best answer, though given the question seems to be focusing on a workaround...
i'm mistaken about setting a system property, instead you have to override the gwt-provided JettyLauncher type and pass it to dev mode as an argument. Something like this in your project (this is not client code, but you also don't want it shipping with your server...)
public class DevModeJettyLauncher extends JettyLauncher {
@Override
protected WebAppContext createWebAppContext(TreeLogger logger, File appRootDir) {
WebAppContext webAppContext = super.createWebAppContext(logger, appRootDir);
webAppContext.setAttribute("org.eclipse.jetty.server.webapp.WebInfIncludeJarPattern", "none");//this is just a regex that matches nothing
return webAppContext;
}
}
add to args something like -server com.company.project.server.dev.DevModeJettyLauncher