74818 [flutter_tools] delete old directories when unzipping ontop of them (cla: yes, tool)

74827 Replace MockXcode with Xcode.test in more unit tests (cla: yes, tool)

74829 [flutter_tools] rewrite Usage.test to capture objects instead of print logs (cla: yes, tool)

74832 [flutter_tools] opt all flutter tool libraries and tests out of null safety. (cla: yes, team, tool)

74834 Replace MockProcessManager with FakeProcessManager in process_test where possible (cla: yes, tool)

74838 Swap MockStdio to FakeStdio in tests (cla: yes, team, tool)

74844 Make flutter create --sample null-safe (cla: yes, tool, waiting for tree to go green)