samples/poppopwin: upgrade to first-class sample

samples/pop-pop-win/test/ppw/test_game.dart

 part of ppw_test;
 
 class TestGame {
   static void run() {
     group('Game', () {
       test('initial values', _testInitial);
       test('setFlag', _testSetFlag);
       test('cannot reveal flagged', _testCannotRevealFlagged);
       test('cannot flag revealed', _testCannotFlagRevealed);
       test('reveal zero', _testRevealZero);
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           expect(g.getSquareState(x,y), equals(SquareState.revealed));
         }
         expect(g.state, isNot(equals(GameState.reset)));
         expect(g.state, isNot(equals(GameState.lost)));
       }
     }
 
     expect(g.state, equals(GameState.won));
   }
 }