Do implementers have to implement private members?

On DartPad, the following code does not compile

```
class A {
int thisIsPublic;
int _thisIsPrivate;
}

class B implements A {
@override
int thisIsPublic;
}
```

The error is `The non-abstract class 'B' is missing implementations for these members: - A._thisIsPrivate`.

Do implementers have to implement private members?

Reproduce the DartPad example and inspect the language specification sections covering implements clauses and private members. Determine whether A._thisIsPrivate is intentionally required of B, then document or correct the specified behavior and add a corresponding language test if the repository identifies one.

Written by the indexing model from the issue text.