1. override
void debugFillDescription(List<String> description)

Accumulates a list of strings describing the current node's fields, one field per string. Subclasses should override this to have their information included in toStringDeep.

Source 

@override
void debugFillDescription(List<String> description) {
  // call to ${super.debugFillDescription(prefix)} is omitted because the root superclasses don't include any interesting information for this class
  assert(() {
    description.add('debug mode enabled - ${Platform.operatingSystem}');
    return true;
  });
  description.add('window size: ${ui.window.physicalSize} (in physical pixels)');
  description.add('device pixel ratio: ${ui.window.devicePixelRatio} (physical pixels per logical pixel)');
  description.add('configuration: $configuration (in logical pixels)');
  if (ui.window.semanticsEnabled)
    description.add('semantics enabled');
}