@Test public void flatteningToMap_emptyMaps() {

  ImmutableList<Translation> translations =
      ImmutableList.of(new Translation(ImmutableMap.of()), new Translation(ImmutableMap.of()));
  Map<Integer, String> merged = translations.stream()
      .map(Translation::dictionary)
      .collect(flattening(Map::entrySet, toMap()));
  assertThat(merged).isEmpty();
}