anchor.diff:  
   void main() {
     testWidgets('StaggeredGridView - ', (WidgetTester tester) async {
  +    final MediaQueryData data = new MediaQueryData.fromWindow(ui.window);
  +    final Size screenSize = data.size;
  +    const int crossAxisCount = 4;
  +    double cellLength = screenSize.width / crossAxisCount;
  +
  +
       final List<int> log = <int>[];
       final widgets = new List<Widget>.generate(20, (int i){
         return new Builder(
  
positive.code: 
      final ScrollableState scrollableState = tester.state(find.byType(Scrollable));
      final ScrollPosition scrollPosition = scrollableState.position;
      scrollPosition.jumpTo(1000.0);
  
      expect(log, isEmpty);
      await tester.pump();
  
      expect(log, equals(<int>[
        6, 7, 8, 9,
  
negative.code:         child: new StaggeredGridView(
              crossAxisSpacing: 0.0,
              mainAxisSpacing: 0.0,
              crossAxisCount: crossAxisCount,
              staggeredTiles: tiles,
              children: widgets,
          ),
        )
      );
  
      List<Offset> expectedTopLeftNormalizedOffsets = const <Offset>[
        const Offset(0.0, 0.0),
        const Offset(2.0, 0.0),
        const Offset(3.0, 0.0),
        const Offset(2.0, 1.0),
  
        const Offset(0.0, 2.0),
  
        const Offset(0.0, 3.0),
  
        const Offset(0.0, 4.0),
        const Offset(1.0, 4.0),
        const Offset(2.0, 4.0),
        const Offset(3.0, 4.0),
  
        const Offset(0.0, 5.0),
        const Offset(1.0, 5.0),
        const Offset(2.0, 5.0),
        const Offset(3.0, 5.0),
  
        const Offset(3.0, 6.0),
        const Offset(2.0, 7.0),
        const Offset(3.0, 7.0),
        const Offset(1.0, 8.0),
        const Offset(2.0, 8.0),
        const Offset(3.0, 8.0),
      ];
  
      for (var item in log) {
        expect(tester.getTopLeft(find.text('$item')), equals
          (expectedTopLeftNormalizedOffsets[item] * cellLength));
        expect(tester.getSize(find.text('$item')), equals(_getTileSize
          (tiles[item],
            cellLength)));
      }
  
  