By Amy Dain Quick quiz: What is the biggest growth district in Massachusetts outside of the city of Boston right now? Did you guess Kendall Square? Cambridge Crossing? Somerville’s Union Square? Watertown’s Arsenal Street? Quincy Center? Lynn’s waterfront? Revere Beach? Woburn’s Commerce Way? Waltham along Route 128? By my read, it is a place without an identity or united governance, a district that straddles the Mystic and Malden Rivers at the intersection of six cities: Everett, Medford, Somerville, Chelsea, Malden, and also a bit of Boston, at Charlestown. The area made headlines recently for the proposed Sky Everett, a 21-story residential tower. Not many places allow that kind of height. And for decades, Everett did not have the market to support that kind of height. Now, Everett is not alone rising to the sky. Right across the Mystic, in Somerville’s Assembly Row, a roof deck graces the 24th floor of a new 826-residence tower. Just down the river, a 29-story tower of 695 homes is proposed for Charlestown.