anchor.diff:    expect(g.bombsLeft, equals(12));
     expect(g.state, equals(GameState.started));
   
  -  expect(() => g.reveal(0, 0), throwsException);
  +  expect(() => g.reveal(0, 0), _throwsAssertionError);
   }
   
   void _testCannotFlagRevealed() {
  
positive.code: import 'package:test/test.dart';
  
  import 'test_util.dart';
  
  void main() {
    test('initial values', _testInitial);
  
negative.code:   expect(g.getSquareState(1, 1), equals(SquareState.revealed));
    expect(g.state, equals(GameState.started));
  
    expect(() => g.setFlag(1, 1, true), _throwsAssertionError);
  }
  
  void _testLoss() {
  