Testing package level Dart code from the outside

lib_greeter.dart

library greeter;

part 'src/greeter.dart';

lib_src_greeter.dart

part of greeter;

class Greeter {
  String _phrase = 'Hello';
  
  String greet(String name) => '$_phrase, $name!';
}

test_greeter_test.dart

library greeter;

part '../lib/src/greeter.dart';

main() {
  final greeter = new Greeter();
  
  greeter._phrase = 'Testphrase';
  
  if (greeter.greet('Me') != 'Testphrase, Me!')
    throw 'Test failed';
}