Prohibit `extends` on public abstract classes with private abstract members.

[AlexanderFarkas]

This is valid:

abstract class A {
  void _performImpl();

  void perform() {
    ...
    _performImpl();
    ...
  }
}

While extending this class from other file is always invalid.

[srawlins]

One note: Extending A from within the library where A is declared can be valid.

[lrhn]

The issues with library private members in public interfaces is known. We have no current plans to change that.

This is a particularly useless case, where it might be reasonable to warn the user that the class can be implemented.
I'd recommend not making the class public, rather than introduce rules preventing you from extending the class.

We hope that adding the ability to prevent implementation or extension of classes explicitly (#2242) will allow you to disallow it in the cases where you want to, without the compiler having to figure out whether a use is safe or not.

[AlexanderFarkas]

When you introduceclosed modifier for classes, lint rule no_non_closed_public_abstract_classes_with_private_abstract_members will be awesome