Matrix4.skew(double alpha, double beta)

Skew matrix around X axis (alpha) and Y axis (beta).

Source 

factory Matrix4.skew(double alpha, double beta) {
  final m = new Matrix4.identity();
  m._m4storage[1] = Math.tan(beta);
  m._m4storage[4] = Math.tan(alpha);
  return m;
}