signedArea function
The signed area of a closed ring, anticlockwise positive.
Implementation
double signedArea(List<Vector2> r) {
var a = 0.0;
for (var i = 0; i < r.length; i++) {
final p = r[i], q = r[(i + 1) % r.length];
a += p.x * q.y - q.x * p.y;
}
return a / 2;
}