assertThat(unmodifiable.asMap().get("bar")).containsExactly(5, -1);
    assertNull(unmodifiable.asMap().get("missing"));

    assertFalse(unmodifiable.entries() instanceof Serializable);
  }

  /** Prepares the multimap for unmodifiable tests, returning an unmodifiable view of the map. */
  private static Multimap<@Nullable String, @Nullable Integer> prepareUnmodifiableTests(
      Multimap<@Nullable String, @Nullable Integer> multimap,