code: @GwtIncompatible("Unnecessary")
    private Writer openExternExportsStream(B options, String path) throws IOException {
      if (options.externExportsPath == null) {
        return null;
      }
  
      String exPath = options.externExportsPath;
  
      if (!exPath.contains(File.separator)) {
        File outputFile = new File(path);
        exPath = outputFile.getParent() + File.separatorChar + exPath;
      }
  
      return fileNameToOutputWriter2(exPath);
    }
docs: Returns a stream for outputting the generated externs file.
  
  @param options The options to the Compiler.
  @param path The path of the generated JS source file.
  @return The stream or null if no extern-ed exports are being generated.