testWidgets('test', (WidgetTester tester) async {
  await tester.pumpWidget(new MyApp());

  // Get the bytes of the screenshot
  RenderRepaintBoundary repaintRenderBoundary = tester
      .allRenderObjects
      .firstWhere((RenderObject object) => object is RenderRepaintBoundary);
  ui.Image image = await repaintRenderBoundary.toImage();
  ByteData bytes = await image.toByteData(format: ui.ImageByteFormat.png);

  // Get the bytes of the golden file
  File golden = new ui.File('/path/to/golden.png');
  List<int> goldenBytes = await golden.readAsBytes();

  // Fail the test if the screenshot isn't within tolerance of golden
  ...
});