code: public static <K, V> KeyValueSink<K, V> asMapSink(ImmutableMultimap.Builder<K, V> multimapB) {
      return new ImmutableMultimapBuilderSink<>(multimapB);
    }
docs: Returns a wrapper around an {@link com.google.common.collect.ImmutableMultimap.Builder} which
  hides the differences between it and {@link com.google.common.collect.ImmutableMap.Builder} for
  the purpose of population.