code: public File buildOne(String filename, File dir) throws Exception {
  
      String dist = "dev";
      final MediaType type;
      AssetWriter writer;
      if (scripts.test(filename)) {
        type = MediaType.js;
        writer = new AssetWriter(dist, null, dir, ".js", ";", charset, this.fileset, scripts);
      } else if (styles.test(filename)) {
        type = MediaType.css;
        writer = new AssetWriter(dist, null, dir, ".css", "", charset, this.fileset, styles);
      } else {
        return null;
      }
  
      List<AssetProcessor> pipeline = pipeline(dist);
      compile(pipeline, ImmutableList.of(filename).iterator(), type, writer, path -> true, 0, 0);
  
      return new File(dir, filename);
    }
docs: Apply the processor pipeline to the given asset. Like {@link #build(String, File)} but for a
  single file or asset.
  
  @param filename Asset to build.
  @return Processed asset.
  @throws Exception If something goes wrong.