75259 [flutter_tools] use TestFeatureFlags instead of MockFeatureFlags (cla: yes, tool, waiting for tree to go green)

75263 [flutter_tools] remove mockito from flutter symbolize test (cla: yes, tool)

75264 Re-land “Migrate driver tests in example/ to NNBD (#75022)” (cla: yes, d: examples, team, waiting for tree to go green)

75265 Re-land “Migrate some benchmarks to NNBD (75023)”  (cla: yes, team, waiting for tree to go green)

75266 Revert “Make flutter_driver be usable from a null-safe app” (a: tests, cla: yes, framework)