@Test public void flatteningToMap_unique() {

  ImmutableList<Translation> translations = ImmutableList.of(
      new Translation(ImmutableMap.of(1, "one")), new Translation(ImmutableMap.of(2, "two")));
  Map<Integer, String> merged = translations.stream()
      .map(Translation::dictionary)
      .collect(flattening(Map::entrySet, toMap()));
  assertThat(merged)
      .containsExactly(1, "one", 2, "two")
      .inOrder();
}