TypeChecker.isAssignableFromType does not consider nullability

I would expect `TypeChecker.isAssignableFromType(staticType)` to take the nullability of the types into account. It returns `true` incorrectly if the TypeChecker type is not nullable but `staticType` is nullable.

Dart SDK version: 2.16.1

Locate TypeChecker.isAssignableFromType and reproduce the nullable staticType case described in the issue, using the referenced Dart SDK version if available. The work is complete when a non-nullable TypeChecker type is not considered assignable from a nullable staticType, with coverage for the nullability behavior.

Written by the indexing model from the issue text.