code: private void checkFirstModule(List<JSModule> modules) {
      if (modules.isEmpty()) {
        report(JSError.make(EMPTY_MODULE_LIST_ERROR));
      } else if (modules.get(0).getInputs().isEmpty() && modules.size() > 1) {
        // The root module may only be empty if there is exactly 1 module.
        report(JSError.make(EMPTY_ROOT_MODULE_ERROR,
            modules.get(0).getName()));
      }
    }
docs: Verifies that at least one module has been provided and that the first one
  has at least one source code input.