Refactor MovedToPluginWarningPlugin to es6 (#3789)

* refactor of lib/MovedToPluginWarningPlugin to es6 syntax

lib/MovedToPluginWarningPlugin.js

@@ -2,18 +2,20 @@
 	MIT License http://www.opensource.org/licenses/mit-license.php
 	Author Tobias Koppers @sokra
 */
-function MovedToPluginWarningPlugin(optionName, pluginName) {
-	this.optionName = optionName;
-	this.pluginName = pluginName;
+"use strict";
+module.exports = class MovedToPluginWarningPlugin {
+	constructor(optionName, pluginName) {
+		this.optionName = optionName;
+		this.pluginName = pluginName;
+	}
+	apply(compiler) {
+		const optionName = this.optionName;
+		const pluginName = this.pluginName;
+		compiler.plugin("compilation", (compilation) => {
+			compilation.warnings.push(new Error `webpack options:
+			DEPRECATED option ${optionName} will be moved to the ${pluginName}. 
+			Use this instead.
+			For more info about the usage of the ${pluginName} see https://webpack.github.io/docs/list-of-plugins.html`);
+		});
+	}
 }
-module.exports = MovedToPluginWarningPlugin;
-
-MovedToPluginWarningPlugin.prototype.apply = function(compiler) {
-	var optionName = this.optionName;
-	var pluginName = this.pluginName;
-	compiler.plugin("compilation", function(compilation) {
-		compilation.warnings.push(new Error("webpack options:\nDEPRECATED option '" + optionName + "' will be moved to the " + pluginName + ". " +
-			"Use this instead.\n" +
-			"For more info about the usage of the " + pluginName + " see https://webpack.github.io/docs/list-of-plugins.html"));
-	});
-};