Dart-使用'is'关键字奇怪的行为进行运行时类型检查

Question

使用is关键字检查运行时类型时，在dart中发现了一个奇怪的行为。检查对象是否为某种类型后，可以将其称为方法和字段。不幸的是，这在全局变量的情况下会出现问题。

---

这是一个指示问题的示例：

复制此代码并将其粘贴到DartPad，以查看错误：https://www.dartpad.dev/

class A{
  final aVal = 10;
}

class B extends A{
  final bVal = 5;
}

void main() {}

void withParam(A a){
  if(a is B){
    a..aVal..bVal; // no error here
  }
}

final A a = B();
void withGlobalScope(){
  if(a is B){    
    // If it checked that 'a' is instance of 'B' why it has no 'bVal' property?
    // Why does dart compiler signals this as an error?
    // Using the 'as' keyword to cast variable 'a' to type 'B' would solve the issue of course, but it seems unnecessary
    a..aVal..bVal; // The getter 'bval' isn't defined for the type 'A'
  }
}

有关类型转换和签入飞镖的文档：https://dart.dev/guides/language/language-tour#type-test-operators