code: @GwtIncompatible("Unnecessary")
    private List<SourceFile> createExternInputs(List<String> files) throws IOException {
      List<FlagEntry<JsSourceType>> externFiles = new ArrayList<>();
      for (String file : files) {
        externFiles.add(new FlagEntry<JsSourceType>(JsSourceType.EXTERN, file));
      }
      try {
        return createInputs(externFiles, false, new ArrayList<JsModuleSpec>());
      } catch (FlagUsageException e) {
        throw new FlagUsageException("Bad --externs flag. " + e.getMessage());
      }
    }
docs: Creates JS extern inputs from a list of files.