addMixinsTo method
Adds all mixins used in this message and any submessages.
Implementation
void addMixinsTo(Set<PbMixin> output) {
  if (mixin != null) {
    output.addAll(mixin!.findMixinsToApply());
  }
  for (var m in _messageGenerators) {
    m.addMixinsTo(output);
  }
}