74788 fix for upcoming lint use_named_constants (a: accessibility, a: tests, cla: yes, d: examples, f: cupertino, f: material design, framework, team, tool)

74798 Catch cases in tests where people try to tap on widgets that aren’t reachable (a: accessibility, a: tests, cla: yes, f: cupertino, f: material design, framework, team, waiting for tree to go green)

74820 Remove “unnecessary” imports in examples (cla: yes, d: examples, team, waiting for tree to go green)

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

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