Boston Ballet's big bow
Photo: Daniel BlueLast Saturday marked not only the official beginning of Boston Ballet's 50th Anniversary Season, but also a big step up for the company in terms of the city's popular consciousness: the Ballet threw a giant, free dance party (okay, officially its "Night of Stars" gala dance concert) that drew upwards of 55,000 people to the Common (above) to see for themselves how far Boston's dance ambassadors to t…