use native implementations of Set and Map (fix #607)

lib/single-file/single-file-core.js

@@ -21,10 +21,15 @@
  *   Source.
  */
 
+/* global window */
+
 this.singlefile.lib.core = this.singlefile.lib.core || (() => {
 
 	const DEBUG = false;
 
+	const Set = window.Set;
+	const Map = window.Map;
+
 	let util, cssTree;
 
 	function getClass(...args) {