[dart2js] Handle NullableType from createFromStaticType
[dart2js] Handle NullableType from createFromStaticType

A NullableType can be a precise AbstactValue if the base type can be
precise.

The main effect is more lowering of field setters to HFieldSet,
(i.e. for fields with nullable types.

Change-Id: If1f92e1a79da87a73df02da3b9fc8fc3542f7b40
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139326
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>
Commit-Queue: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk