List<FileRef> files = Lists.newArrayList();
    files.addAll(gxpFiles);
    files.addAll(extraFiles());
    // compile
    CompilationSet compilationSet = getCompilationSetBuilder().build(files);
    AlertSink alertSink = createAlertSink();
    compilationSet.compile(alertSink, getAlertPolicy(), getOutputLanguages(),
                           getAllowedOutputPredicate(gxpFiles));
    return compilationSet;
  }