ES6 shim subclassability

[bergus]

Not sure to what extent this is desired.
Some problems with the ES6-like interface are:

• methods (including static ones) don't return subclass instances
• static methods don't use this to resolve their inputs (and don't require this to be a promise constructor at all)

We might want to put up a warning for this. Not sure how complicated or performance-deteriorating it would be if we wanted to support this.

[briancavalier]

Good question. I don't intend to support it. I'd rather creed stay smaller and not require all the hoop jumping. So, yeah, I think a warning is the best thing.