void scale(num scaleX, num scaleY)

Source 

void scale(num scaleX, num scaleY) {
  _data[0] = this.a * scaleX;
  _data[1] = this.b * scaleY;
  _data[2] = this.c * scaleX;
  _data[3] = this.d * scaleY;
  _data[4] = this.tx * scaleX;
  _data[5] = this.ty * scaleY;
}