This is how I'm trying to create a lazy loaded service.

gistfile1.txt

$rollbarServiceDefinition = new \Symfony\Component\DependencyInjection\Definition('\RollbarNotifier');
$rollbarServiceDefinition->addArgument($config['rollbarnotifier']);
$rollbarServiceDefinition->setLazy(true);
$container->setDefinition('foo.service.rollbarnotifier', $rollbarServiceDefinition);

... later on when I do a $container->get('foo.service.rollbarnotifier'), I see it invoking my __construct() function of \RollbarNotifier