use set lookup to keep track of loaded modes

lib/constants.js

@@ -300,14 +300,14 @@ const unfold = (f, seed) => {
 }`
 
 if (typeof window !== 'undefined' && typeof window.navigator !== 'undefined') {
-  const required = []
+  const alreadyLoaded = new Set()
 
   LANGUAGES.filter(language => language.mode !== 'auto').forEach(language => {
-    if (language.mode && required.indexOf(language.mode) === -1) {
-      required.push(language.mode)
-      !language.custom
-        ? require(`codemirror/mode/${language.mode}/${language.mode}`)
-        : require(`./customModes/${language.mode}`)
+    if (language.mode && !alreadyLoaded.has(language.mode)) {
+      alreadyLoaded.add(language.mode)
+      language.custom
+        ? require(`./customModes/${language.mode}`)
+        : require(`codemirror/mode/${language.mode}/${language.mode}`)
     }
   })
 }