Composing `sealed` classes

Is there currently any way to compose sealed classes in Dart? I would like to have the ability to construct a sealed class hierarchy using another already declared sealed hierarchy.

For example, consider the following code snippet:
```dart
sealed class A {}

class A1 extends A {}

class A2 extends A {}

// A can be instantiated with A1 or A2
// A a = A1 | A2

sealed class B keyword A {} // keyword or another syntax

class B1 extends B {}

class B2 extends B {}

// B = A | B1 | B2
// which should expand to:
// B = A1 | A2 | B1 | B2

void main() {
A a = A1(); // Fine
A a2 = B1(); // Error
B b = A1(); // Fine
B b2 = B1(); // Fine
}
```

I don't want to change the `sealed class A` hierarchy.

The issue contains a language-design proposal but names no implementation file, test, or entry point. Start by reviewing the sealed-class example and the existing Dart rules for class hierarchies; done would require an accepted syntax and specified semantics for composing A with B1 and B2 without changing A.

Written by the indexing model from the issue text.