code: public static <K, V> KeyValueSink<K, V> asMapSink(ImmutableMap.Builder<K, V> mapB) {
      return new ImmutableMapBuilderSink<>(mapB);
    }
docs: Returns a wrapper around an {@link com.google.common.collect.ImmutableMap.Builder} which
  hides the differences between it and {@link com.google.common.collect.ImmutableMultimap.Builder} for
  the purpose of population. Beware {@link com.google.common.collect.ImmutableMap.Builder} will
  still throw an exception for duplicate entries!