- The deleted chunk in TFBlock has been unreachable since 57ef23ef. The
RemoveComments change would make it reachable by dropping the leading
Null in a task or function body.
- The optimization in TFBlock was unsound because the nested block could
shadow identifiers used (but not defined) in the tail of the outer
block.
- The optimization is not clearly necessary, so I'm dropping it for now.
- Expand tf_block test coverage.