static @Nullable String[] concat(@Nullable String[] first, @Nullable String[] second) {
    if (first == null)
      return second;
    if (second == null)
      return first;
    return ObjectArrays.concat(first, second, String.class);
  }
Element used mostly for error reporting,
real model provided by {@link #sourceElement()}.
value-processor/src/org/immutables/value/processor/meta/Proto.java