Fix when tests for TS 3.9 (#44160)
Fix when tests for TS 3.9 (#44160)

TS 3.9 forbids deleting a property that isn't optional or undefined. One
of when's tests deletes a property like this, so I added a cast to make
it compile.
markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped