Previously we would use the value of the `nullability` getter as
the carry over nullablity for the substituted type. This CL changes
that to `declaredNullability`. The change is observable in
substituted extension types: their `nullability` is a combination
of the representation type nullability and the declared nullability,
and its value range includes `Nullability.undetermined`, which should
not be available for `ExtensionType.declaredNullability`.
The unexpected value for `ExtensionType.declaredNullability` was
detected by the assert in
`_NullabilityMarkerDetector.visitExtensionType`.
Change-Id: I38f81df57db18ec94f2f6422134c02efe6649be6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/350323
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>
Chloe Stefantsova
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