[analyzer] add warning when abstract interface class has unused implementation.

[mmcdon20]

@lrhn mentioned in dart-lang/language#3736:

The analyzer should be able to tell you if an abstract interface class has implementation, and it's never extended inside the same library. If it doesn't today, it's at least a possibility.

Consider the following example:

abstract interface class Vehicle {
  final String make;
  final String model;
  Vehicle(this.make, this.model);
  
  void moveForward() {
    print('$make $model is moving forward!');
  }
}

If there are no classes in the same library that extend this interface then the implementation, the body of moveForward and the constructor Vehicle are effectively unreachable code. The analyzer should provide a warning about this, but does not currently.