[jashkenas]

Ah yes -- this would be very against the open/dynamic spirit of CS and JS -- and for that reason, we'd never add it. Many valid uses of subtypes don't need to implement every method defined by a parent type (or interface) in order to be used correctly.

For example, a rich "collection" interface that has some helper functions for key:value hash-like collections, but that a more array- or set-like subtype doesn't have to implement.

If you forget to implement a method that you later try to use, you'll find out when you try to use it. Such is the nature of the beast.