Bug: Webpack process and setImmediate polyfill interferes with time slicing

[gaearon]

Here's the copy paste of the polyfill: https://gist.github.com/gaearon/5d2f7bedf76d7bbec2b5c1ee04861ce3

This reproes with old CRA (e.g. react-scripts@0.7.0). I saw this on this branch: Swizec/react-fractals#16

The result is concurrent renders get grouped into a huge blocking task:

Not sure why yet.

[gaearon]

In CRA 4 there's also a setImmediate polyfill but at least it uses MessageChannel so it's not as bad.