I’m using CycleJs, which uses RxJs. The dist/rx.all.js looks like this:
;(function(undefined) {...}.call(this));
Webpack is rewriting it as this:
;(function(undefined) {...}).call(undefined);
Which causes “this” to be undefined when the function is executed, which makes stuff break because of “use strict”
For future readers, I was using babel-loader because my stuff is ES6. It looks like that was the problem, because excluding node_modules made the issue go away.