Add a hint `@nonVirtual` for class members

… or, to help developers when defining the intended use of a class, and to help users use a class properly (i.e. as intended by the class). I’d find this extremely useful when working on fairly complex classes with lots and lots of member fields and methods.

Similar to the spirit of @sealed (#27372):

import 'package:meta/meta.dart';

class X {
  @nonVirtual
  void alwaysPrintFoo() {
    print('Foo');
  }
}

class Y extends B {
  // HINT: X.alwaysPrintFoo is marked @nonVirtual and should not be overridden.
  void alwaysPrintFoo() {
    print('Bar');
  }
}

General concept:

• Issue a hint when a member annotated with @nonVirtual is overriden.

Because of how this annotation would need to work, I imagine it would also need to:

• Issue a hint when a class with one or more @nonVirtual members is implemented (implements – not extends/with), as by definition a class with @nonVirtual member requires the original implementation:

  import 'package:meta/meta.dart';
  
  class X {
    @nonVirtual
    void alwaysPrintFoo() {
      print('Foo');
    }
  }
  
  // HINT: Cannot implement a class with one or more `@nonVirtual` members.
  abstract class Y implements X {}

• Issue a hint when an abstract member is annotated with @nonVirtual. Also impossible:

  import 'package:meta/meta.dart';
  
  abstract class X {
    // HINT: Cannot define a `@nonVirtual` abstract member.
    @nonVirtual
    void alwaysPrintFoo();
  }

/cc @davidmorgan @srawlins for thoughts.