Apply more let/const transforms.

_isKeyable.js

@@ -6,7 +6,7 @@
  * @returns {boolean} Returns `true` if `value` is suitable, else `false`.
  */
 function isKeyable(value) {
-  var type = typeof value;
+  const type = typeof value;
   return (type == 'string' || type == 'number' || type == 'symbol' || type == 'boolean')
     ? (value !== '__proto__')
     : (value === null);