Convert the DOMCMapReaderFactory to an ES6 class

Given that we only create *one* instance of this class per `getDocument` call, this shouldn't matter performance wise.

src/display/dom_utils.js

@@ -52,22 +52,20 @@ DOMCanvasFactory.prototype = {
   }
 };
 
-var DOMCMapReaderFactory = (function DOMCMapReaderFactoryClosure() {
-  function DOMCMapReaderFactory(params) {
-    this.baseUrl = params.baseUrl || null;
-    this.isCompressed = params.isCompressed || false;
+class DOMCMapReaderFactory {
+  constructor({ baseUrl = null, isCompressed = false, }) {
+    this.baseUrl = baseUrl;
+    this.isCompressed = isCompressed;
   }
 
-  DOMCMapReaderFactory.prototype = {
-    fetch(params) {
-      var name = params.name;
+  fetch({ name, }) {
     if (!name) {
       return Promise.reject(new Error('CMap name must be specified.'));
     }
     return new Promise((resolve, reject) => {
-        var url = this.baseUrl + name + (this.isCompressed ? '.bcmap' : '');
+      let url = this.baseUrl + name + (this.isCompressed ? '.bcmap' : '');
 
-        var request = new XMLHttpRequest();
+      let request = new XMLHttpRequest();
       request.open('GET', url, true);
 
       if (this.isCompressed) {
@@ -78,7 +76,7 @@ var DOMCMapReaderFactory = (function DOMCMapReaderFactoryClosure() {
           return;
         }
         if (request.status === 200 || request.status === 0) {
-            var data;
+          let data;
           if (this.isCompressed && request.response) {
             data = new Uint8Array(request.response);
           } else if (!this.isCompressed && request.responseText) {
@@ -100,11 +98,8 @@ var DOMCMapReaderFactory = (function DOMCMapReaderFactoryClosure() {
 
       request.send(null);
     });
-    },
-  };
-
-  return DOMCMapReaderFactory;
-})();
+  }
+}
 
 /**
  * Optimised CSS custom property getter/setter.

test/unit/test_utils.js

@@ -16,20 +16,19 @@
 import { CMapCompressionType } from '../../src/shared/util';
 
 class NodeCMapReaderFactory {
-  constructor(params) {
-    this.baseUrl = params.baseUrl || null;
-    this.isCompressed = params.isCompressed || false;
+  constructor({ baseUrl = null, isCompressed = false, }) {
+    this.baseUrl = baseUrl;
+    this.isCompressed = isCompressed;
   }
 
-  fetch(params) {
-    var name = params.name;
+  fetch({ name, }) {
     if (!name) {
       return Promise.reject(new Error('CMap name must be specified.'));
     }
     return new Promise((resolve, reject) => {
-      var url = this.baseUrl + name + (this.isCompressed ? '.bcmap' : '');
+      let url = this.baseUrl + name + (this.isCompressed ? '.bcmap' : '');
 
-      var fs = require('fs');
+      let fs = require('fs');
       fs.readFile(url, (error, data) => {
         if (error || !data) {
           reject(new Error('Unable to load ' +