Apply more let/const transforms.

uniqueId.js

@@ -1,7 +1,7 @@
 import toString from './toString.js';
 
 /** Used to generate unique IDs. */
-var idCounter = 0;
+let idCounter = 0;
 
 /**
  * Generates a unique ID. If `prefix` is given, the ID is appended to it.
@@ -21,7 +21,7 @@ var idCounter = 0;
  * // => '105'
  */
 function uniqueId(prefix) {
-  var id = ++idCounter;
+  const id = ++idCounter;
   return toString(prefix) + id;
 }