That genius Lucian Wischik at Microsoft has written a nice tool called Mr Flakey which you can use to fiddle with async calls.
He describes the use in detail in the link, but essentially Mr Flakey lets you simulate async calls failing (e.g. network errors) and test how your code responds to these errors. It works with both Windows 8.1 and Windows Phone 8.1, so it’s perfect for universal apps.
This is something I can already see being incredibly useful and I’m expecting it to be a useful tool for app development for quite some time.