reasons for using var ？

[cyphernull]

react/packages/scheduler/src/forks/SchedulerDOM.js

Line 253 in f15f8f6

function unstable_runWithPriority(priorityLevel, eventHandler) {

Is there a certain reason for using var instead of let in these unstale_xxx functions and other variables' definition ? 🙂

[rickhanlonii]

The scheduler is designed to be used with no build steps, which means using ES5 syntax.

See #13582 for more context.