code: function(p){
          var normal = this.b.sub(this.a).perpendicular();
          var d = p.sub(this.a).dot(normal);
          return mathUtils.sign(d);
      }
docs: Computes the dot product of these 2 vectors: line start -> point
  and the perpendicular line direction if the result is negative.
  
  @param {Vec2D} p
  @return classifier Number