Pixar’s “Cars 2” overcame surprisingly negative critical reviews to overperform at the weekend box office, selling an estimated $68 million at North American theaters. That No. 1 finish extends Pixar’s perfect box-office track record — all 12 of its feature films have opened in first place. How sales hold up, however, will be watched carefully. “Cars 2,” directed by John Lasseter, chief creative officer at Pixar, represents an end to the studio’s perfect critical track record; only about 34 percent of reviews were positive, according to RottenTomatoes.com, by far the worst for any Pixar title.