refactor classes, add private, minor fixes

1-js/11-async/02-promise-basics/head.html

@@ -0,0 +1,13 @@
+<script>
+function loadScript(src) {
+  return new Promise(function(resolve, reject) {
+    let script = document.createElement('script');
+    script.src = src;
+
+    script.onload = () => resolve(script);
+    script.onerror = () => reject(new Error("Script load error: " + src));
+
+    document.head.append(script);
+  });
+}
+</script>