Webpack will randomly execute script order if its runtime is not prioritized before chunks execute.
This seems to be somehow triggered in #13870 because of slightly different script ordering.
This had actually broke CSS, which is why our tests are failing 50% of the time:
Without this PR:
However, it's still problematic to use `async` in development since we rely on script execution order. So, this PR disables `async` in development.
We're exploring `defer` in the future anyway (over `async`), which will be ordered, so I don't mind diverging between dev and prod in this way.
---
Fixes#13911
d874329d11
