Build: Update jsdom; migrate a test with Symbol polyfill to an iframe test

So far, we've been testing that jQuery element iteration works with polyfilled
Symbol & transpiled for-of via a Node test with jsdom with the Symbol global
removed. Unfortunately, jsdom now requires Symbol to be present for its internal
functionality so such a test is no longer possible. Instead, it's been migrated
to an iframe test with transpiled JavaScript.

This PR also enables us to use ECMAScript 2017 or newer in Node.js code.

Closes gh-4305

2 files changed, 48 insertions, 55 deletions

diff --git a/test/promises_aplus_adapters/deferred.js b/test/promises_aplus_adapters/deferred.js
index 31af16655..5e3ffe2d8 100644
--- a/test/promises_aplus_adapters/deferred.js
+++ b/test/promises_aplus_adapters/deferred.js
@@ -1,20 +1,17 @@
 "use strict";
 
-require( "jsdom" ).env( "", function( errors, window ) {
-	if ( errors ) {
-		console.error( errors );
-		return;
-	}
+const { JSDOM } = require( "jsdom" );
 
-	var jQuery = require( "../../" )( window );
+const { window } = new JSDOM( "" );
 
-	module.exports.deferred = function() {
-		var deferred = jQuery.Deferred();
+const jQuery = require( "../../" )( window );
 
-		return {
-			promise: deferred.promise(),
-			resolve: deferred.resolve.bind( deferred ),
-			reject: deferred.reject.bind( deferred )
-		};
+module.exports.deferred = () => {
+	const deferred = jQuery.Deferred();
+
+	return {
+		promise: deferred.promise(),
+		resolve: deferred.resolve.bind( deferred ),
+		reject: deferred.reject.bind( deferred )
 	};
-} );
+};
diff --git a/test/promises_aplus_adapters/when.js b/test/promises_aplus_adapters/when.js
index adde1a13a..3e945d475 100644
--- a/test/promises_aplus_adapters/when.js
+++ b/test/promises_aplus_adapters/when.js
@@ -1,49 +1,45 @@
 "use strict";
 
-require( "jsdom" ).env( "", function( errors, window ) {
-	if ( errors ) {
-		console.error( errors );
-		return;
-	}
+const { JSDOM } = require( "jsdom" );
 
-	var jQuery = require( "../../" )( window );
+const { window } = new JSDOM( "" );
 
-	module.exports.deferred = function() {
-		var adopted, promised,
-			obj = {
-				resolve: function() {
-					if ( !adopted ) {
-						adopted = jQuery.when.apply( jQuery, arguments );
-						if ( promised ) {
-							adopted.then( promised.resolve, promised.reject );
-						}
-					}
-					return adopted;
-				},
-				reject: function( value ) {
-					if ( !adopted ) {
-						adopted = jQuery.when( jQuery.Deferred().reject( value ) );
-						if ( promised ) {
-							adopted.then( promised.resolve, promised.reject );
-						}
-					}
-					return adopted;
-				},
+const jQuery = require( "../../" )( window );
 
-				// A manually-constructed thenable that works even if calls precede resolve/reject
-				promise: {
-					then: function() {
-						if ( !adopted ) {
-							if ( !promised ) {
-								promised = jQuery.Deferred();
-							}
-							return promised.then.apply( promised, arguments );
-						}
-						return adopted.then.apply( adopted, arguments );
-					}
+module.exports.deferred = () => {
+	let adopted, promised;
+
+	return {
+		resolve: function() {
+			if ( !adopted ) {
+				adopted = jQuery.when.apply( jQuery, arguments );
+				if ( promised ) {
+					adopted.then( promised.resolve, promised.reject );
 				}
-			};
+			}
+			return adopted;
+		},
+		reject: function( value ) {
+			if ( !adopted ) {
+				adopted = jQuery.when( jQuery.Deferred().reject( value ) );
+				if ( promised ) {
+					adopted.then( promised.resolve, promised.reject );
+				}
+			}
+			return adopted;
+		},
 
-		return obj;
+		// A manually-constructed thenable that works even if calls precede resolve/reject
+		promise: {
+			then: function() {
+				if ( !adopted ) {
+					if ( !promised ) {
+						promised = jQuery.Deferred();
+					}
+					return promised.then.apply( promised, arguments );
+				}
+				return adopted.then.apply( adopted, arguments );
+			}
+		}
 	};
-} );
+};