ES6 refactor of parserStore.js (#6083)

const, method shorthands, for of, arrow functions, ES6 Map

lib/dialects/parserStore.js

 'use strict';
 
-var stores = {};
-module.exports = function (dialect) {
-  stores[dialect] = stores[dialect] || {};
+const stores = new Map();
+
+module.exports = dialect => {
+
+  if (!stores.has(dialect)) {
+    stores.set(dialect, new Map());
+  }
 
   return {
-    clear: function () {
-      stores[dialect] = {};
+    clear() {
+      stores.get(dialect).clear();
     },
-    refresh: function (dataType) {
-      dataType.types[dialect].forEach(function (type) {
-        stores[dialect][type] = dataType.parse;
-      });
+    refresh(dataType) {
+      for (const type of dataType.types[dialect]) {
+        stores.get(dialect).set(type, dataType.parse);
+      }
     },
-    get: function (type) {
-      return stores[dialect][type];
+    get(type) {
+      return stores.get(dialect).get(type);
     }
   };
 };