78836 Make version and time in flutter_tool null safe (a: null-safety, cla: yes, team, tool)

78843 Make language_version in flutter_tool null safe (a: null-safety, cla: yes, team, tool)

78845 Migrate flutter_tool tool helper files to null safety (a: null-safety, cla: yes, team, tool)

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)