2-7-mixim-1.dart

void main() {
  var sh = CustomShape();
  sh.getBorder(sh.borders);
}

class CustomShape extends Shape with BorderHelper {
  CustomShape(): super([1.0, 2.0]);
}



mixin BorderHelper {
  void getBorder(List<double> borders) {
    print(borders.length);
  }
  //int get bordersCount => (border.)
}

class Shape {
  final List<double> borders;
  Shape(this.borders);
}


class Trapezoid extends Shape {
  Trapezoid(): super([8.0, 5.0, 10.0, 5.0]);
}

class Country {
  final List<double> borders;
  Country(this.borders);
}

void getBorder(BorderHelper helper) {
  //print(helper.bordersCount);
}

все ок. зачет