Apply let/const transform.

_charsEndIndex.js

@@ -10,7 +10,7 @@ import baseIndexOf from './_baseIndexOf.js';
  * @returns {number} Returns the index of the last unmatched string symbol.
  */
 function charsEndIndex(strSymbols, chrSymbols) {
-  var index = strSymbols.length;
+  let index = strSymbols.length;
 
   while (index-- && baseIndexOf(chrSymbols, strSymbols[index], 0) > -1) {}
   return index;