Stub node fs module in browser.
fs.readFile is still a function, would not throw an error.
Only when user calls fs.readFile(), it then throw an error.
By default, browserify/webpack/parcel/fusebox/dumber would stub fs module with nothing or an empty object {}.
I was confused why @babel/core v7.8.0+ does not work in browser anymore. It turns out the babel uses gensync and that checks the type of sync and errback options. It rejects if they are not function.
This new fs stub bypasses the issue.
From https://github.com/babel/babel/blob/master/packages/babel-core/src/gensync-utils/fs.js
export const readFile = gensync<[string, "utf8"], string>({
sync: fs.readFileSync,
errback: fs.readFile,
});