strategy.dart

abstract class PrintStrategy {
  String print(String string);
}

class Printer {
  PrintStrategy _strategy;
  Printer({PrintStrategy strategy}) : _strategy = strategy;

  String print(String string) {
    return _strategy.print(string);
  }
}

class UpperCaseStrategy extends PrintStrategy {
  @override
  String print(String string) {
    return string.toUpperCase();
  }
}

class LowerCaseStrategy extends PrintStrategy {
  @override
  String print(String string) {
    return string.toLowerCase();
  }
}

main(List<String> arguments) {
  var lower = Printer(strategy: new LowerCaseStrategy());
  lower.print("O tempora, o mores!");

  var upper = Printer(strategy: new UpperCaseStrategy());
  upper.print("O tempora, o mores!");
}