double aspectRatio

The aspect ratio to attempt to use.

The aspect ratio is expressed as a ratio of width to height. For example, a 16:9 width:height aspect ratio would have a value of 16.0/9.0.

Source 

double get aspectRatio => _aspectRatio;
void aspectRatio=(double newAspectRatio)

Source 

set aspectRatio (double newAspectRatio) {
  assert(newAspectRatio != null);
  assert(newAspectRatio > 0.0);
  assert(newAspectRatio.isFinite);
  if (_aspectRatio == newAspectRatio)
    return;
  _aspectRatio = newAspectRatio;
  markNeedsLayout();
}