Fix let back to const

test/unit/dom-repeat.html

@@ -3935,7 +3935,7 @@ suite('chunked rendering', function() {
   let chunked;
   let notifyChange;
   let targetCount;
-  let handleChange = () => {
+  const handleChange = () => {
     if (!targetCount || chunked.$.repeater.renderedItemCount >= targetCount) {
       notifyChange(Array.from(chunked.root.querySelectorAll('*:not(template):not(dom-repeat)')));
     }