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

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

74857 Migrate remaining examples to null-safety (cla: yes, d: examples, team, waiting for tree to go green)

75022 Migrate driver tests in example/ to NNBD (cla: yes, d: examples, team, waiting for tree to go green)

75161 Revert “Migrate driver tests in example/ to NNBD” (cla: yes, d: examples, team)