更新：这是一个文档错误，已修复：https : //github.com/dart-lang/test/pull/1471
根据测试包（https://pub.dev/packages/test）的文档/示例，此测试用例应该可以工作并且不会触发警告。但是它确实：
The non-nullable local variable 'b' must be assigned before it can be used.
Try giving it an initializer expression, or ensure that it's assigned on every execution path.dart(not_assigned_potentially_non_nullable_local_variable)
将变量标记为后期作品，但我想在我提交一个错误说文档错误之前检查我没有遗漏任何东西。=)
The non-nullable local variable 'b' must be assigned before it can be used.
Try giving it an initializer expression, or ensure that it's assigned on every execution path.dart(not_assigned_potentially_non_nullable_local_variable)