Bug 1391405: Part 4 - Avoid easily-avoidable regexp. r=zombie

MozReview-Commit-ID: 6xAr5hGxm2r

--HG--
extra : rebase_source : eef7eb12615faaf6fa9adc87685ef74be9413a0c
extra : histedit_source : 0c1eb852ea025f8eb9c75dc15706da6f88cb475b

toolkit/components/extensions/Schemas.jsm

@@ -1187,7 +1187,7 @@ class Type extends Entry {
     }
 
     let choice;
-    if (/^[aeiou]/.test(type)) {
+    if ("aeiou".includes(type[0])) {
       choice = `be an ${type} value`;
     } else {
       choice = `be a ${type} value`;