rotate method
Implementation
Angle rotate(Angle theta, [bool anticlockwise = true]) {
if (!anticlockwise) theta = -theta;
final x = (this.x * theta.cosValue) - (this.y * theta.sinValue);
final y = (this.x * theta.sinValue) + (this.y * theta.cosValue);
return Angle.fromXY(x, y);
}