@Test public void mergingValues_emptyMaps() {

  ImmutableList<Translation> translations =
      ImmutableList.of(new Translation(ImmutableMap.of()), new Translation(ImmutableMap.of()));
  Map<Integer, String> merged = translations.stream()
      .map(Translation::dictionary)
      .collect(flatMapping(BiStream::from, groupingBy((Integer k) -> k, (a, b) -> a + "," + b)))
      .collect(ImmutableMap::toImmutableMap);
  assertThat(merged).isEmpty();
}