Add some consistency checks

Author: ilevkivskyi

mypy/nodes.py

@@ -597,6 +597,10 @@ def is_trivial_self(self) -> bool:
 
     @property
     def setter(self) -> Decorator:
+        # Do some consistency checks first.
+        first_item = self.items[0]
+        assert isinstance(first_item, Decorator)
+        assert first_item.var.is_settable_property
         assert self.setter_index is not None
         item = self.items[self.setter_index]
         assert isinstance(item, Decorator)