chore: fix `require_trailing_commas` #2474

Job	Run time
examples (packages/flutter_bloc/example)	57s
examples (examples/flutter_bloc_with_stream)	1m 17s
examples (examples/flutter_complex_list)	1m 21s
examples (examples/flutter_counter)	1m 40s
examples (examples/flutter_dynamic_form)	1m 24s
examples (examples/flutter_firebase_login)	2m 6s
examples (examples/flutter_form_validation)	1m 2s
examples (examples/flutter_infinite_list)	1m 18s
examples (examples/flutter_login)	1m 27s
examples (examples/flutter_shopping_cart)	1m 34s
examples (examples/flutter_timer)	1m 28s
examples (examples/flutter_todos)	1m 46s
examples (examples/flutter_todos/packages/local_storage_todos_api)	1m 7s
examples (examples/flutter_todos/packages/todos_api)	1m 14s
examples (examples/flutter_todos/packages/todos_repository)	56s
examples (examples/flutter_weather)	1m 39s
examples (examples/flutter_wizard)	1m 6s
examples (examples/github_search/flutter_github_search)	1m 16s
build	1s
	24m 39s

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