private function addServiceInstance($id, Definition $definition, $isSimpleInstance)
    {
        $class = $this->dumpValue($definition->getClass());

        if (0 === strpos($class, "'") && false === strpos($class, '$') && !preg_match('/^\'(?:\\\{2})?[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*(?:\\\{2}[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*)*\'$/', $class)) {
            throw new InvalidArgumentException(sprintf('"%s" is not a valid class name for the "%s" service.', $class, $id));
        }

        $isProxyCandidate = $this->getProxyDumper()->isProxyCandidate($definition);
        $instantiation = '';

        if (!$isProxyCandidate && $definition->isShared()) {
            $instantiation = "\$this->services['$id'] = ".($isSimpleInstance ? '' : '$instance');
        } elseif (!$isSimpleInstance) {
            $instantiation = '$instance';
        }

        $return = '';
        if ($isSimpleInstance) {
            $return = 'return ';
        } else {
            $instantiation .= ' = ';
        }

        $code = $this->addNewInstance($definition, $return, $instantiation, $id);

        if (!$isSimpleInstance) {
            $code .= "\n";
        }

        return $code;
    }