code: private void compile(AssetInfo asset, File source, File target) throws ServiceException {
          File watched = watchedAssets.get(asset);
          if (watched == null) {
              if (isDevMode()) {
                  new Thread(() -> {
                      try {
                          // avoid recursive compiles
                          if (isDevMode())
                              watchedAssets.put(asset, target);
                          doCompile(asset, source, target);
                      }
                      catch (Exception ex) {
                          logger.severeException(ex.getMessage(), ex);
                      }
                  }).start();
              }
              else {
                  doCompile(asset, source, target);
              }
          }
      }
docs: Returns null except in dev mode.