Thanks so much for approving the previous PR (#139048) a couple weeks ago!
This one is the same, except it's covering files in `examples/` and `packages/flutter/lib/src/animation/`.
(solving issue #136139)
This is a proof of concept for renaming SlottedMultiChildRenderObjectWidgetMixin to SlottedMultiChildRenderObjectWidget and making it a concrete class.
I also made SlottedContainerRenderObjectMixin generic instead of being specialized to RenderBox.
I don't think this is something we can easily automigrate, but we may not need to, I don't know how common this is...
