Also:
- don't mark a node as from the new generation if it is dirty, since we
know it still has to be built.
- establish the rule that you can't call setState() during initState()
or build().
- make syncChild() return early for unchanged children.
- update the tests, including adding a new one.
If it's a StatefulComponent, then it's ok to reuse it so long as it
hasn't been initialised.
If it's a regular Component or a TagNode, then it's always ok to reuse.
If it's a RenderObjectWrapper, then it's ok to reuse so long as it
doesn't have a renderObject.
To put it another way, this changes how we prevent the following
nonsensical situations from arising:
- Sync two stateful StatefulComponents together
- Sync two RenderObjectWrappers with RenderObjects together
When either of those cases happen, we just drop the old one on the
ground and use the new one unchanged.
