Webpack warning when using targetPlatform "auto" and webpack target "web"

[adamjmcgrath]

Hi, I created a Gist of how to reproduce the webpack warning: https://gist.github.com/adamjmcgrath/fbc6b1a0991f98fa9ca13bef92bc825b

When you run rollup -c && webpack rolled-up.js you get:
Module not found: Error: Can't resolve 'worker_threads' in '/dev/tmp-rollup-webpack'

It's essentially when you target web in webpack.

I'm testing our output against a Gatsby build that will do one webpack build for node (to generate the html) and one for web to generate the JavaScript.