allow a string to be passed as the optional option

CHANGELOG.md

@@ -11,6 +11,11 @@
 
 _Note: Gaps between patch versions are faulty/broken releases._
 
+## 2.11.3
+
+ * **Bug Fix**
+  * Allow a string to be passed as the `optional` option.
+
 ## 2.11.2
 
  * **Bug Fix**

lib/6to5/file.js

@@ -72,6 +72,7 @@ File.normaliseOptions = function (opts) {
 
   opts.blacklist = util.arrayify(opts.blacklist);
   opts.whitelist = util.arrayify(opts.whitelist);
+  opts.optional  = util.arrayify(opts.optional);
 
   _.defaults(opts, {
     moduleRoot: opts.sourceRoot