Found files excluded from sound null safety

[devoncarew]

Found files excluded from sound null safety (see
also dart-lang/sdk#49169):

test/rules/null_closures_test.dart:23:// @dart=2.9
test/rules/null_closures_test.dart:31:// @dart=2.9
test/rules/null_closures_test.dart:41:// @dart=2.9
test/rules/null_closures_test.dart:51:// @dart=2.9
test/rules/prefer_collection_literals_test.dart:23:// @dart=2.9    
test/rules/prefer_collection_literals_test.dart:32:// @dart=2.9    
test/rules/prefer_collection_literals_test.dart:41:// @dart=2.9    
test_data/integration/always_require_non_null_named_parameters/lib.dart:5:// @dart=2.9
test_data/integration/avoid_private_typedef_functions/lib.dart:5:// @dart=2.9
test_data/integration/avoid_private_typedef_functions/part.dart:5:// @dart=2.9
test_data/integration/cancel_subscriptions/cancel_subscriptions.dart:5:// @dart=2.9
test_data/integration/close_sinks/src/a.dart:5:// @dart=2.9
test_data/integration/only_throw_errors/only_throw_errors.dart:5:// @dart=2.9
test_data/integration/overridden_fields/src/a.dart:5:// @dart=2.9
test_data/integration/overridden_fields/src/b.dart:5:// @dart=2.9
test_data/integration/public_member_api_docs/test/b.dart:5:// @dart=2.9
test_data/integration/use_build_context_synchronously/lib/unmigrated.dart:5:// @dart=2.9
test_data/rules/always_require_non_null_named_parameters.dart:7:// @dart=2.9
test_data/rules/avoid_init_to_null.dart:5:// @dart=2.9
test_data/rules/avoid_returning_null.dart:7:// @dart=2.9
test_data/rules/avoid_returning_null_for_future.dart:7:// @dart=2.8
test_data/rules/prefer_bool_in_asserts.dart:5:// @dart=2.9
test_data/rules/void_checks.dart:7:// @dart=2.9