Since none of the subparts of the base method share any local state,
make the base method virtual and override it in all of the subclasses
with the subclass-specific functionality.
Also add an `ASSERT(IsNull());` check to all base implementations to
cause an error in DEBUG mode when forgetting to override in a subclass,
and then perform what should happen in the null case (if anything).
This way, in DEBUG mode, it's easier to distinguish an accidental null
slipping through vs. a lack of overriding.
TEST=Pure refactoring, so existing tests.
Change-Id: I57a669e5eb34119443d84bf1208ce7232a0e4c6c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/262347
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>
Tess Strickland
82897fdfde