refactor(es6) upgrade MultiWatching to ES6 class

lib/MultiWatching.js

@@ -2,22 +2,24 @@
 	MIT License http://www.opensource.org/licenses/mit-license.php
 	Author Tobias Koppers @sokra
 */
-var async = require("async");
+"use strict";
 
-function MultiWatching(watchings) {
-	this.watchings = watchings;
+const async = require("async");
+
+class MultiWatching {
+	constructor(watchings) {
+		this.watchings = watchings;
+	}
+
+	invalidate() {
+		this.watchings.forEach((watching) => watching.invalidate());
+	}
+
+	close(callback) {
+		async.forEach(this.watchings, (watching, finishedCallback) => {
+			watching.close(finishedCallback);
+		}, callback);
+	}
 }
 
-MultiWatching.prototype.invalidate = function() {
-	this.watchings.forEach(function(watching) {
-		watching.invalidate();
-	});
-};
-
-MultiWatching.prototype.close = function(callback) {
-	async.forEach(this.watchings, function(watching, finishedCallback) {
-		watching.close(finishedCallback);
-	}, callback);
-};
-
 module.exports = MultiWatching;