Vector3D.angleBetween() vs Math.acos() + Vector3D.dotProduct() methods

Vector3D.angleBetween() method returns NaN in some cases, for example with Vector3D(1, 1, 1) and Vector3D(-1, -1, -1).  Evaluating cos between two vectors and getting angle with Math.acos() method is to avoid NaN.  This code compares speed of these two calculations.

日本語の解説 (explanation in Japanese):
http://www.fumiononaka.com/TechNotes/Flash/FN1104001.html