num scaleX

The horizontal scale (percentage) of the object as applied from the pivot point.

1.0 equals 100% scale.

Scaling the local coordinate system changes the x and y property values, which are defined in whole pixels.

Source 

@override
num get scaleX => _scaleX;
void scaleX=(num value)

Source 

@override
set scaleX(num value) {
  if (value is num) _scaleX = value;
  _transformationMatrixRefresh = true;
}