method rotate

Source

void rotate(num rotation) {

  var cosR = cos(rotation);
  var sinR = sin(rotation);

  var a =  this.a;
  var b =  this.b;
  var c =  this.c;
  var d =  this.d;
  var tx = this.tx;
  var ty = this.ty;

  _data[0] = a * cosR - b * sinR;
  _data[1] = a * sinR + b * cosR;
  _data[2] = c * cosR - d * sinR;
  _data[3] = c * sinR + d * cosR;
  _data[4] = tx * cosR - ty * sinR;
  _data[5] = tx * sinR + ty * cosR;
}