code: @CheckReturnValue
    public ErrorProneFlags plus(ErrorProneFlags other) {
      Map<String, String> combinedMaps = new HashMap<>(this.getFlagsMap());
      combinedMaps.putAll(other.getFlagsMap());
      return ErrorProneFlags.fromMap(combinedMaps);
    }
docs: Returns a new ErrorProneFlags object with the values of two ErrorProneFlags objects added
  together. For flags that appear in both instances, the values in {@code other} override {@code
  this}.