80763 Migrate gen_l10n to null safety (team, tool, cla: yes, waiting for tree to go green, a: null-safety)

80916 Move FakeOperatingSystemUtils from context.dart to fakes.dart (team, tool, cla: yes, waiting for tree to go green, a: null-safety)

81002 Migrate analyze_size to null safety (team, tool, cla: yes, a: null-safety)

81006 Migrate forbidden_from_release_tests to null safety (team, cla: yes, waiting for tree to go green, a: null-safety)

81352 Replace MockAndroidDevice and MockIOSDevice with fakes (team, tool, cla: yes, waiting for tree to go green, a: null-safety)