How to use the @jupyterlab/buildutils.WPPlugin function in @jupyterlab/buildutils

/*-----------------------------------------------------------------------------
| Copyright (c) Jupyter Development Team.
| Distributed under the terms of the Modified BSD License.
|----------------------------------------------------------------------------*/

var plib = require('path');
var fs = require('fs-extra');
var Handlebars = require('handlebars');
var HtmlWebpackPlugin = require('html-webpack-plugin');
var webpack = require('webpack');
var DuplicatePackageCheckerPlugin = require('duplicate-package-checker-webpack-plugin');
var BundleAnalyzerPlugin = require('webpack-bundle-analyzer')
  .BundleAnalyzerPlugin;

var Build = require('@jupyterlab/buildutils').Build;
var WPPlugin = require('@jupyterlab/buildutils').WPPlugin;
var package_data = require('./package.json');

// Handle the extensions.
var jlab = package_data.jupyterlab;
var extensions = jlab.extensions;
var mimeExtensions = jlab.mimeExtensions;
var packageNames = Object.keys(mimeExtensions).concat(Object.keys(extensions));

// Ensure a clear build directory.
var buildDir = plib.resolve(jlab.buildDir);
if (fs.existsSync(buildDir)) {
  fs.removeSync(buildDir);
}
fs.ensureDirSync(buildDir);

// Build the assets