/**
 * Adds `finally` to ES6 `Promise`.
 *
 *     server.listen()
 *       .then( () => {
 *         // do something
 *       })
 *       .finally( () => {
 *         // always perform clean up
 *       });
 *
 */
interface Promise<T> {
    finally(fn: () => void): Promise<T>;
}