ExtractTextPlugin不输出css

Question

使用Webpack 3的此配置似乎不会在public/css中输出任何CSS：

import path from 'path';
import CleanWebpackPlugin from 'clean-webpack-plugin';
import ExtractTextPlugin from 'extract-text-webpack-plugin';

const ep = new ExtractTextPlugin({filename: ["./public/css/app.css"]});

export default {
    context: path.resolve(__dirname, 'resources', 'assets', 'js'),
    entry: './app.js',
    output: {
        filename: 'app.js',
        path: path.resolve(__dirname, 'public', 'js')
    },
    module: {
        rules: [
            {
                test: /\.js$/,
                include: /resources\/assets\/js/,
                exclude: /node_modules/,
                use: {
                    loader: "babel-loader",
                    options: {
                        presets: ['env']
                    }
                }
            },
            {
                test: /app.scss$/,
                include: [path.resolve(__dirname, 'resources', 'assets', 'sass')],
                exclude: [path.resolve(__dirname, "node_modules")],
                use: ep.extract({
                    use: [
                        {
                            loader: 'sass-loader',
                            options: {
                                includePaths: [
                                    path.resolve(__dirname, 'node_modules', 'foundation-sites', 'scss')
                                ],
                                sourceMap: false
                            }
                        }
                    ],
                    fallback: "style-loader"
                })
            }
        ]
    },
    plugins: [
        new CleanWebpackPlugin([
            path.resolve(__dirname, 'public', 'js'),
            path.resolve(__dirname, 'public', 'css')
        ]),
        ep
    ]
};

packages.json：

{
  "private": true,
  "devDependencies": {
    "babel-cli": "^6.26.0",
    "babel-loader": "^7.1.2",
    "babel-preset-env": "^1.6.1",
    "babel-register": "^6.26.0",
    "clean-webpack-plugin": "^0.1.17",
    "extract-text-webpack-plugin": "^3.0.2",
    "gulp": "^3.9.1",
    "laravel-elixir-browserify-official": "^0.1.3",
    "node-sass": "^4.7.2",
    "sass-loader": "^6.0.6",
    "webpack": "^3.10.0"
  },
  "dependencies": {
    "@fortawesome/fontawesome": "^1.1.2",
    "@fortawesome/fontawesome-free-brands": "^5.0.4",
    "@fortawesome/fontawesome-pro-light": "^5.0.4",
    "@fortawesome/fontawesome-pro-regular": "^5.0.4",
    "fontfaceobserver": "^2.0.9",
    "foundation-sites": "^6.4.3",
    "jquery": "^3.3.0",
    "jquery-recliner": "^0.2.5",
    "laravel-elixir": "^6.0.0-15"
  },
  "scripts": {
    "build": "./node_modules/.bin/webpack",
    "build:prod": "./node_modules/.bin/webpack -p",
    "watch": "./node_modules/.bin/webpack --watch"
  }
}