ToplineThe U.S.’ bluechip stock index is on the cusp of suffering its longest stretch in the red since 1978, a potentially concerning signal for investors enjoying an otherwise sanguine period for stocks, though Tuesday’s losses are largely tied to a singular company—UnitedHealth Group—and gains remain healthy over a longer timeframe.December has not been festive for the Dow.AFP via Getty Images Key FactsThe Dow Jones Industrial Average fell 350 points, or 0.8%, by mid afternoon, heading to its ninth-consec ...