[whatwg] Optional non-blocking mode for simple dialogs (alert, confirm, prompt).

[Ben Rimmington]

<http://www.whatwg.org/specs/web-apps/current-work/multipage/timers.html#simple-dialogs>

An optional callback parameter could be added to the simple dialogs API:

> result = window.alert(message, [callback]);
> 
> result = window.confirm(message, [callback]);
> 
> result = window.prompt(message, [default], [callback]);

If the callback argument is recognised as a function value:

  * the result is the supplied callback;

  * the callee returns without waiting for user input;

  * the callback is invoked after the dialog is dismissed.

Otherwise [see current spec]:

  * the callee simply ignores the callback argument;

  * the script is paused until the dialog is dismissed;

  * the result is undefined, boolean, string or null.

For user agents that don't support the updated API:

> function promptWrapper(message, default, callback) {
>     var result = window.prompt(message, default, callback);
>     if (callback && (typeof callback === "function")) {
>         if (result !== callback) {
>             callback(result);
>         }
>         return callback;
>     }
>     return result;
> }

I'm not sure if navigator.yieldForStorageUpdates() is needed in the non-blocking mode.