18345 Revert “Re-enable Fuchsia tests (#18342)” (cla: yes)

18346 Introduce TaskRunnerAffineWeakPtrFactory to generate TaskRunnerAffineWeakPtrs (cla: yes)

18347 Roll src/third_party/dart d6fed1f62444..29c00e28f350 (16 commits) (cla: yes, waiting for tree to go green)

18348 null-annotate lerp.dart, annotations.dart, channel_buffers.dart, hash_codes.dart (cla: yes)

18349 null-annotate lib/ui/natives.dart (cla: yes)