Whether Lockheed Martin stock is a buy depends in large part on sales of its F-35.Lockheed Martin's (LMT -0.17%) F-35 Lightning II stealth fighter jet is the gift that keeps on giving. Estimated to be worth 1trillionintotallifetimevalue,theF−35warplaneremainseasilyLockheed′smostimportantproduct.Andthiswasdemonstratedonceagainjustlatelastyear,whentheweekbeforeChristmassawthePentagondeliverthreegiftstoLockheedMartinworth15.5 billion in total.Two gifts for Lockheed Mar ...