Treating createSentinel as returning a value of type Never caused issues
whenever its result flowed into a union, since the union would just
simplify to its other component. This meant that if a late field started
out uninitialized and was later initialized to a constant, we would see
it as only ever having the constant value. This caused isSentinel to
always return false even if the field hadn't been initialized yet.
Instead, we support a late sentinel value in the abstract value domain
and use this value as the return type of createSentinel during
inference. Additionally, inference treats _lateReadCheck as a simple
narrowing to exclude the late sentinel value.
Change-Id: I086bfd77576930e3ca2a4cfc9bd63476b6636685
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/210646
Reviewed-by: Stephen Adams <sra@google.com>
