anchor.diff:  import 'package:test/test.dart';
   
   import 'test_util.dart';
  +
  +final _throwsAssertionError = throwsA(const TypeMatcher<AssertionError>());
   
   void main() {
     test('initial values', _testInitial);
  
positive.code:   expect(g.bombsLeft, equals(12));
    expect(g.state, equals(GameState.started));
  
    expect(() => g.reveal(0, 0), throwsException);
  }
  
  void _testCannotFlagRevealed() {
  
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() {
  