27362 NNBD migration for scenario_app (cla: yes)

27364 Revert “NNBD migration for scenario_app” (cla: yes, needs tests)

27365 Scenario nnbd (cla: yes, needs tests)

27366 Roll Dart SDK from 2b81a20ef985 to 3c094fbf2093 (1 revision) (cla: yes, waiting for tree to go green)

27367 Fix dart analysis (cla: yes, waiting for tree to go green, needs tests)

27368 Switch test_suites to yaml. (affects: tests, cla: yes, platform-fuchsia)

27370 refactor and simplify CI dart analysis (cla: yes, waiting for tree to go green)