Address encapsulation challenges in abstract classes

When transitioning from other programming languages to Dart, I occasionally find myself in a situation where I'd like to define an abstract class with hidden implementation methods. These methods are intended to be implemented differently in concrete classes, but they shouldn't be exposed as part of the public API; they are meant to be treated as implementation details.

I've been searching for a workaround to achieve the desired level of encapsulation:

`abstract.dart`
```dart
abstract class Abstract {
int _aInt();
int aInt() => _aInt();
}
```

`concrete.dart`
```dart
class Concrete extends Abstract {
@override
int _aInt() => 1; // The method doesn’t override an inherited method.
}
```

`main.dart`
```dart
void main() {
// NoSuchMethodError: tried to call a non-function, such as null: 'core.NoSuchMethodError.withInvocation'
print(Concrete().aInt());
}
```

Is anyone aware of a workaround for this issue in Dart? I would greatly appreciate any guidance or suggestions.

Start with the abstract.dart, concrete.dart, and main.dart examples in the issue, then read the Dart language specification sections covering abstract classes, private members, and method overriding. Determine whether the requested encapsulation is supported or requires a language change; done means a documented resolution or a clearly scoped proposal.

Written by the indexing model from the issue text.