@Test public void mergingValues_uniqueKeys() {

  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(flatMapping(BiStream::from, groupingBy((Integer k) -> k, (a, b) -> a + "," + b)))
      .collect(ImmutableMap::toImmutableMap);
  assertThat(merged)
      .containsExactly(1, "one", 2, "two")
      .inOrder();
}