LONDON — Anthony Joshua survived the first knockdown of his young career, coming back to drop Wladimir Klitschko twice before stopping him in the 11th round Saturday night to retain his heavyweight titles and stake his claim as the boxing’s next star. Before a boisterous crowd of 90,000 at Wembley Stadium, Joshua and Klitschko traded huge punches in a spectacular fifth round and again in the sixth before Joshua finally found a way to finish the longtime former champion late in the fight. The biggest heavyweight fight in more than a decade had something for everyone, but in the end it was Joshua who announced himself as the future of the division.