use consistent event for moon browser XHR (#259)

In the HTTP driver, the `onload` event is used rather than
`addEventListener`. This PR refactors Moon Browser to use the same
method by using `onload` along with an arrow function to clean up the
event handling code that adds the listener.

packages/moon-browser/dist/moon-browser.js

@@ -1254,7 +1254,8 @@
 				load();
 			} else {
 				var xhr = new XMLHttpRequest();
-				xhr.addEventListener("load", function () {
+
+				xhr.onload = function () {
 					if (xhr.readyState === xhr.DONE) {
 						if (xhr.status === 0 || xhr.status === 200) {
 							var _scriptNew = document.createElement("script");
@@ -1268,7 +1269,8 @@
 						script.parentNode.removeChild(script);
 						load();
 					}
-				});
+				};
+
 				xhr.open("GET", src, true);
 				xhr.send();
 			}

packages/moon-browser/src/index.js

@@ -28,7 +28,7 @@ function load() {
 		} else {
 			const xhr = new XMLHttpRequest();
 
-			xhr.addEventListener("load", function() {
+			xhr.onload = () => {
 				if (xhr.readyState === xhr.DONE) {
 					if (xhr.status === 0 || xhr.status === 200) {
 						const scriptNew = document.createElement("script");
@@ -41,7 +41,8 @@ function load() {
 					script.parentNode.removeChild(script);
 					load();
 				}
-			});
+			};
+
 			xhr.open("GET", src, true);
 			xhr.send();
 		}

packages/moon-browser/test/browser.test.js

@@ -5,15 +5,14 @@ let status = 200;
 
 window.Moon = Moon;
 window.XMLHttpRequest = function() {
-	this.addEventListener = (type, fn) => {
+	this.open = () => {};
+	this.send = () => {
 		this.readyState = 0;
-		fn();
+		this.onload();
 
 		this.readyState = 1;
-		fn();
+		this.onload();
 	};
-	this.open = () => {};
-	this.send = () => {};
 
 	this.DONE = 1;