Address null safety exclusions in flutter/packages

See #110016 for context:

convert fuchsia_ctl to null safety: Delete fuchsia_ctl package packages#2528
convert packages/imitation_game/imitation_tests/smiley/flutter/smiley to null safety: Migrate imitation_game and pigeon to null safety packages#2526
convert packages/pigeon/platform_tests/android_unit_tests, .../ios_swift_unit_tests, and .../ios_unit_tests to null safety: Migrate imitation_game and pigeon to null safety packages#2526
convert imitation_game to null safety: [imitation_game] Migrate imitation game to null safety packages#2483
convert web_benchmarks to null safety: Migrate package [web_benchmarks] to null safety #98647

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