Manual Dart roll fa4379946109467c8a48f20f19d83d7c72968a3e...d45c3d15cb3cea0104a87697c085259666eec528 (#13761)

dart-lang/sdk@d45c3d15cb [VM/nnbd] Make Nullability and NNBDMode class enums to avoid name conflicts.
dart-lang/sdk@cfec969eff Update dartdoc version to 0.29.1.
dart-lang/sdk@e3e0ef8548 Make explanations for unchanged types optional and disabled by default
dart-lang/sdk@e39b0f8d99 Disallow a deferred import of a library with extensions if any extensions are visible
dart-lang/sdk@9c983d2ee3 [cfe] Support extensions via prefix
dart-lang/sdk@5f5ce5d119 Include type parameter bounds into string expections for subtyping.
dart-lang/sdk@dc69edcdb3 NullabilityEliminator for removing nullabilities in legacy libraries.
dart-lang/sdk@7ad7e6202d Allow extensions imported with a prefix to be accessible
dart-lang/sdk@ea57b1e62c [VM/nnbd] Pass nullability when creating Class::DeclarationType.