Dec 29 - The Dow and S&P 500 closed at all-time highs on Wednesday on a boost from retailers including Walgreens and Nike, as investors shrugged off concerns on the spreading Omicron variant. The Dow has now risen six straight trading days, marking the longest streak of gains since a seven-session run from March 5 to March 15 this year. Walgreens Boots Alliance and Nike Inc rose against the backdrop of recent reports suggesting holiday sales were strong for U.S. retailers. Data on Wednesday s...