Apply even more let/const transforms.

_strictLastIndexOf.js

@@ -9,7 +9,7 @@
  * @returns {number} Returns the index of the matched value, else `-1`.
  */
 function strictLastIndexOf(array, value, fromIndex) {
-  var index = fromIndex + 1;
+  let index = fromIndex + 1;
   while (index--) {
     if (array[index] === value) {
       return index;