Method references are not handled correctly

[kelloggm]

At a high-level, the problem is that a method reference that's not solvable is basically equivalent to a lambda, but unlike a lambda it doesn't come with an arity. So, we have no way to decide what the type should be. In #290, I arbitrarily chose java.util.Supplier<?>, which is correct for the example there (and prevents crashes), but will cause the output not to be compilable if the method reference has a different arity.

I also manually confirmed that if I change the arity, we do have the ability to detect the problem in JavaTypeCorrect (we get a incompatible types: invalid method reference error). I think we can probably solve this problem in a general way by writing code in JavaTypeCorrect to handle that error.