code: function(theta){
          return new Vec2(
              this.x*Math.cos(theta)-this.y*Math.sin(theta), 
              this.x*Math.sin(theta)+this.y*Math.cos(theta)
          );
      }
docs: Rotates the vector in radians counterclockwise and returns a new vector.
  @memberof Vec2#
  @param {Number} theta - rotation angle (in radians)
  @returns {Vec2}