Five years ago, a labor dispute led to a 19-day walkout on Broadway leaving more than two dozen shows dark. In turn, this led to producers and the city of New York losing millions of dollars in potential revenue. That was in 2007, but another potential strike is looming if a deal is not made by the end of the year. This time, it is the 250 cleaners, porters, elevator operators, and bathroom attendants in Broadway’s theaters who are threatening to strike.