Interfaces and base classes are dealt with just as your code shows: explicitly. If your two lines instead read:

var a = new MyThingWithFoo();
var b = new MySpecialIckleSprite();

then the compiler would infer that a should be of type MyThingWithFoo and b of type MySpecialIckleSprite. To override this inference in order to explicitly make the variables by of interface or base class types, then the variable’s type would still be explicitly specified using existing syntax.

However, if I look past my paranoia, I suspect this could actually strengthen the typing in an average AS program ’cause the compiler would start enforcing types after they had been first set.

How do you see interfaces or base classes being dealt with? So things like:

var a :IFoo = new MyThingWithFoo();
var b :Sprite = new MySpecialIckleSprite();