.build();
    CollectorTester.of(collector, equivalence)
        .expectCollects(expected, mapEntry("one", 1), mapEntry("two", 2), mapEntry("three", 3));
  }

  public void testToImmutableSortedMap_exceptionOnDuplicateKey() {
    Collector<Entry<String, Integer>, ?, ImmutableSortedMap<String, Integer>> collector =
        toImmutableSortedMap(Ordering.natural(), Entry::getKey, Entry::getValue);
    assertThrows(