anchor.diff:    ///
     ///     Matrix4 matrix = myNode.transformMatrix;
     Matrix4 get transformMatrix {
  -    if (_transformMatrix != null) {
  -      return _transformMatrix;
  -    }
  -
  +    if (_transformMatrix == null) {
  +      _transformMatrix = computeTransformMatrix();
  +    }
  +    return _transformMatrix;
  +  }
  +
  +  Matrix4 computeTransformMatrix() {
       double cx, sx, cy, sy;
   
       if (_rotation == 0.0) {
  
positive.code:   void set scaleX(double scaleX) {
      assert(scaleX != null);
      _scaleX = scaleX;
      _invalidateTransformMatrix();
    }
  
    /// The vertical scale of this node relative its parent.
  
negative.code:   void set position(Point position) {
      assert(position != null);
      _position = position;
      invalidateTransformMatrix();
    }
  
    /// The draw order of this node compared to its parent and its siblings.
  