protected function installNodePackage( $directory, OutputInterface $output, $package, $version = null, $dev = false ) {
		$package_manager = new Proxy();

		if ( $package_manager->installed( $directory, $package ) ) {
			throw new RuntimeException( 'Package is already installed.' );
		}

		$package_manager->install( $directory, $output, $package, $version, $dev );
	}