public Registry() {}
@Test
  public void registry() {
    List<TestServer> masters = ImmutableList.of(new ServerC(), new ServerB(), new ServerA());
    List<TestServer[]> permutations = new ArrayList<>();
    computePermutations(masters.toArray(new TestServer[0]), 0, permutations);
    // Make sure that the registry orders the masters independently of the order in which they
    // are registered.
    for (TestServer[] permutation : permutations) {
      Registry<TestServer, Void> registry = new Registry<>();
      for (TestServer server : permutation) {
        registry.add(server.getClass(), server);
      }
      Assert.assertEquals(masters, registry.getServers());
    }
  }