AST Transformer API issue with children set at construction

[alessiostalla]

I'll open a PR with a test case. The issue is this: calling withChild with a property that (mistakenly) has no setter causes childrenSetAtConstruction to be true, however, if the node factory's constructor is a lambda provided by the user, children will NOT be set at construction and will be silently ignored. I think we should throw a configuration exception in that case.