Part of files not handled

[jonaskello]

Say I have a file my_lib.dart that specifies two other files using the part directive, part_a.dart and part_b.dart. I can successfully generate part_a_serialization_rules.dart and part_b_serialization_rules.dart using the transformer. However when I try to import these generated files they throw an exception. If I import part_a_serialization_rules.dart it will in turn import part_a.dart which has a "part of my_lib" as a first line which will not work because you cannot import a part by itself.