SOMERVILLE, MASS. — Federal Realty Investment Trust and retailer IKEA have completed a land swap at the Assembly Square retail center in Somerville. As a result of the transaction, Federal Realty now owns an additional 16.6 acres of riverfront property on Mystic River, and IKEA owns 11.9 acres near Interstate 93. Federal Realty plans to add the space to an existing assemblage for the development of a new mixed-use project known as Assembly on the Mystic. IKEA plans to build a new 340,000-square-foot retail location on its parcel and already has infrastructure work under way. The construction timetable for both projects was not released.