What I've found is that you actually want restrictive when you're doing maintenance. You want to know as much as possible about the code you're changing. You want to know exactly what's in that instance variable.
When you're doing prototyping, you want to delay that decision as long as you can, so you are making the most informed decision possible. And nothing informs your decisions like the act of developing and getting feedback from users.
Optional static typing gives you the best of both worlds. But I don't know how you'd get the equivalent with Design By Contract/Unit Testing. Both of those feel a little unnatural to me. I'd rather just tinker, and not have to set up extra stuff before or after I code.
