ActionScript casting objects

Here is the example supplied by Macromedia

function bark(myAnimal:Animal) {
var foo:Dog = Dog(myAnimal);
foo.bark();
}

var curAnimal:Animal = new Dog();
bark(curAnimal); // will work
curAnimal = new Cat();
bark(curAnimal); // won't work


import Dog;
function bark(myAnimal:Animal) {
var foo:Dog = Dog(myAnimal);
if (foo) {
foo.bark();
}
}