Expected Behavior

DataLoader should gracefully fall back to setTimeout if setImmediate is not supported on the browser.

Current Behavior

It crashes with an error ReferenceError: setImmediate is not defined.

Possible Solution

-    setImmediate || setTimeout;
+   typeof setImmediate === 'function' ?
+      setImmediate :
+      (fn => {setTimeout(fn, 0);});

This code also fixes a problem of calling setTimeout without a delay number when it falls back.
Sorry, I didn't create a PR as I am not familiar with the Flow typing that this project is based on.