Repro test for a bug in webpack when having more than one worker

To reproduce:

1. Checkout the code.
2. npm install
3. npm run build
4. Check the generated main*.js files, search for __webpack_require__.u, and notice the following code:

/******/                __webpack_require__.u = (chunkId) => {
/******/                        // return url for filenames based on template
/******/                        return "" + chunkId + "." + undefined + ".js";
/******/                };

That undefined is a problem, because it means the web worker name won't resolve. It should be the hash of the file instead.