In flutter/packages#3020 I fixed some issues in various languages related to Object parameters and return values, ranging from warnings about weird cast syntax to compilation failures. It's very likely that most of the same issues with Object fields. We should add a generic Object field to AllTypes and AllNullableTypes, and fix any resulting issues.

Low priority since having an Object field is a weird thing to do, so this may not come up for anyone in actual usage for quite a while. But we should make sure it works regardless.