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:         child: new StaggeredGridView(
              crossAxisSpacing: 0.0,
              mainAxisSpacing: 0.0,
              crossAxisCount: 4,
              staggeredTiles: tiles,
              children: widgets,
          ),
        )
      );
  
      const double cellLength = 200.0;
  
      expect(tester.getTopLeft(find.text('0')), equals(const Offset(0.0, 0.0)));
      expect(tester.getTopLeft(find.text('1')), equals(const Offset(400.0, 0.0)));
      expect(tester.getTopLeft(find.text('2')), equals(const Offset(600.0, 0.0)));
      expect(tester.getTopLeft(find.text('3')), equals(const Offset(400.0, 200.0)));
  
      expect(tester.getTopLeft(find.text('4')), equals(const Offset(0.0, 400.0)));
      expect(tester.getTopLeft(find.text('5')), equals(const Offset(0.0, 600.0)));
  
      for (var i = 0; i < 6; ++i) {
        expect(tester.getSize(find.text('$i')), equals(_getTileSize(tiles[i],
            cellLength)));
      }
  
  
negative.code: 
      final ScrollableState scrollableState = tester.state(find.byType(Scrollable));
      final ScrollPosition scrollPosition = scrollableState.position;
      scrollPosition.jumpTo(800.0);
  
      expect(log, isEmpty);
      await tester.pump();
  
      for (var item in log) {
        expect(tester.getTopLeft(find.text('$item')), equals
          (expectedTopLeftNormalizedOffsets[item] * cellLength  - const Offset
          (0.0, 800.0)));
        expect(tester.getSize(find.text('$item')), equals(_getTileSize
          (tiles[item],
            cellLength)));
      }
  
      expect(log, equals(<int>[
        6, 7, 8, 9,
  