78892 Migrate command_help to null safety (a: null-safety, cla: yes, team, tool, waiting for tree to go green)

78893 Migrate signals.dart to null safety (a: null-safety, cla: yes, team, tool, waiting for tree to go green)

78896 Migrate flutter_tools file_system to null safety (a: null-safety, cla: yes, team, tool)

78901 Migrate flutter_tools os.dart to null safety (a: null-safety, cla: yes, team, tool)

78911 [flutter_tools] fix deletion of gradle wrapper artifacts in cache (cla: yes, tool, waiting for tree to go green)

78913 [flutter_tools] handle concurrent modification in signal callback (cla: yes, tool)