Walt Disney Co. has announced a sequel to “Frozen,” the film which reinvigorated its animation business. News that “Frozen 2” is in the works comes as little surprise considering the Oscar-winning original made close to $1.3 billion at the worldwide box office, became a merchandising phenomenon and helped push company stock to a 15-year high. The sequel, for which no release date was revealed, was announced Thursday at the company’s annual shareholder meeting in San Francisco. Disney used the same meeting to announce release dates for another major franchise, revealing that “Star Wars: Episode VIII” will be released in May 2017 and the standalone “Star Wars” spin-off film, “Rogue One,” starring Felicity Jones, will be released in December 2016. “Frozen” is the highest-grossing animated movie ever. A new short film featuring the same characters, “Frozen Fever,” will premiere in front of this weekend’s big Disney family release, “Cinderella.” Shares closed up $4.28 or 4.2 percent to $107.17 on the New York Stock Exchange.