update method Null safety
Updates the golden file identified by golden with rendered pixels of
widthxheight.
This will be invoked in lieu of compare when autoUpdateGoldenFiles
is true (which gets set automatically by the test framework when the
user runs flutter test --update-goldens --platform=chrome).
The method by which golden is located and by which its bytes are written
is left up to the implementation class.
Implementation
Future<void> update(double width, double height, Uri golden);