Count comparisons to `null` literal as a nullability signal

This breaks a good deal of tests and overall feels like preventing
a whole bunch of "make it non-nullable" stuff from working.

On the other hand, in cases where not the whole program is migrated
at once, and pieces of it keep being legacy and the whole thing
running in mixed mode, we would like to keep it on the conservative
side, so treat comparisons to `null` as a nullability signal.

I deleted some tests while updating others, I think a good deal of
updated tests probably should be deleted instead, but I kept them
for review, as these diffs show what exactly this change breaks.

Change-Id: I3d29dd446aefb7cfeb6219cffd9060ca8f57e50d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/283681
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Ilya Yanok <yanok@google.com>