code: private void trackMessage(
        NodeTraversal t, JsMessage message, String msgName,
        Node msgNode, boolean isUnnamedMessage) {
      if (!isUnnamedMessage) {
        MessageLocation location = new MessageLocation(message, msgNode);
        messageNames.put(msgName, location);
      } else {
        Var var = t.getScope().getVar(msgName);
        if (var != null) {
          unnamedMessages.put(var, message);
        }
      }
    }
docs: Track a message for later retrieval.
  
  This is used for tracking duplicates, and for figuring out message
  fallback. Not all message types are trackable, because that would
  require a more sophisticated analysis. e.g.,
  function f(s) { s.MSG_UNNAMED_X = 'Some untrackable message'; }