This reverts commit 5fd259be.

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.

This reverts commit c75edc26.

The assert was never hitting because it was first hitting a
null cast error in the `renderer.parent!` line in the for
loop.

* apply upcoming lint cast_nullable_to_non_nullable

* make RenderObjectElement.renderObject non-nullable

Previous benchmarks have established the performance of sync*. Additionally, why allocate a list when you don't need to? Since these APIs are private, they can be re-arranged a bit to avoid creating as many lists. Will probably not have a large effect on benchmarks

* Add punctuation for unterminated sentences in doc comments

* Addressing review nit

* Improve error reporting for RenderObject visitChildren errors

* Nits and fixes