Java-to-Dart codelab: Top-level factory function

main.dart

import 'dart:math';

abstract class Shape {
  num get area;
}

class Circle implements Shape {
  final num radius;
  Circle(this.radius);
  num get area => PI * pow(radius, 2);
}

class Square implements Shape {
  final num side;
  Square(this.side);
  num get area => pow(side, 2);
}

Shape shapeFactory(String type) {
  if (type == 'circle') return new Circle(2);
  if (type == 'square') return new Square(2);
  throw 'Can\'t create $type.';
}

main() {
  var circle = shapeFactory('circle');
  var square = shapeFactory('square');
  print(circle.area);
  print(square.area);
}

Will be replaced by https://gist.github.com/kwalrath/a98247f43448f5e0eac4c7f483c173a5.