fix requestIdleCallback not defined in Edge

routes/_utils/scheduleIdleTask.js

@@ -8,13 +8,19 @@ import { mark, stop } from './marks'
 const taskQueue = new Queue()
 let runningRequestIdleCallback = false
 
+function getRIC () {
+  // we load polyfills asynchronously, so there's a tiny chance this is not defined
+  return typeof requestIdleCallback !== 'undefined' ? requestIdleCallback : setTimeout
+}
+
 function runTasks (deadline) {
   mark('scheduleIdleTask:runTasks()')
   while (taskQueue.length && deadline.timeRemaining() > 0) {
     taskQueue.shift()()
   }
   if (taskQueue.length) {
-    requestIdleCallback(runTasks)
+    let rIC = getRIC()
+    rIC(runTasks)
   } else {
     runningRequestIdleCallback = false
   }
@@ -25,6 +31,7 @@ export function scheduleIdleTask (task) {
   taskQueue.push(task)
   if (!runningRequestIdleCallback) {
     runningRequestIdleCallback = true
-    requestIdleCallback(runTasks)
+    let rIC = getRIC()
+    rIC(runTasks)
   }
 }