Of course, here is the simplest (although perhaps not most efficient) implementation:
function isSubclassOf (a, b) { return new a is b; }
It sucks that in as3, `new a is b` does not imply `a.prototype is b`. as3
seems to take the approach that the prototype of a class is an instance of the
class, but somehow not an instance of the superclass! More bizarrity.
