WebPack で core-js のカスタムビルド結果をくっつける

webpack.config.js

const ConcatSource = require("webpack-sources").ConcatSource;
const corejsBuilder = require("core-js-builder");

function CoreJsBuildPlugin(options) {
    this.options = options || { entry: {} };
}

CoreJsBuildPlugin.prototype.apply = function CoreJsBuildPlugin(compiler) {
    const self = this;

    compiler.plugin("compilation", function (compilation) {
        compilation.plugin("optimize-chunk-assets", function (chunks, callback) {
            corejsBuilder(self.options).then(function (bundledCode) {
                const code = bundledCode;
                chunks.forEach(function (chunk) {
                    if (!chunk.isInitial) return;
                    chunk.files.forEach(function (file) {
                        // 先頭にくっつける
                        compilation.assets[file] = new ConcatSource("/* core-js */\n", code, compilation.assets[file]);
                    });
                });
                callback();
            });
        });
    });
};

//....
module.exports = {
    //....
    plugins: [
        new CoreJsBuildPlugin({  // https://github.com/zloirock/core-js#custom-build-from-external-scripts
            modules: ["es6"],
            blacklist: ["es6.reflect"],  
            library: false,
            umd: true
        })
    ]
}