Update FileLoader.js to use documented way to retrieve XmlHtttpResponse response

src/loaders/FileLoader.js

@@ -168,7 +168,7 @@ Object.assign( FileLoader.prototype, {
 
 			request.addEventListener( 'load', function ( event ) {
 
-				var response = event.target.response;
+				var response = this.response;
 
 				Cache.add( url, response );