Simplify Window.alert IDL

[saschanaz]

https://html.spec.whatwg.org/multipage/window-object.html#the-window-object

interface Window : EventTarget {
  // user prompts
  void alert();
  void alert(DOMString message);
}

How about merging them into void alert(optional DOMString message);?

[TimothyGu]

These two are not compatible. See https://heycam.github.io/webidl/#idl-overloading-vs-union. With the current scheme, alert(undefined) would go to the second overload (with undefined stringified to "undefined"), while using an optional here would make that code function the same way as alert(). Executing alert(undefined) in web browsers seems to indicate that implementations currently use operation overloading.

We could change that behavior of course, but just wanted to make the point clear about compatibility.