This updates the emit for a for..in statement containing a yield when emitting a downlevel generator to more accurately align with EnumerateObjectProperties, which states (emphasis added):

The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration.

This is achieved by adding an extra if (!(key in obj)) continue test at the top of the transformed loop body so that we can skip over deleted properties. We don't make any changes to handle newly added properties since that is spec'd to be implementation defined behavior (per the "not guaranteed" statement in the above condition).

Fixes #49808