Shares of Dropbox Inc. leaped 37 percent Friday in its trading debut, signaling that recent threats to the nine-year-old bull market haven't spilled over to the tech sector. The investment bankers for Dropbox couldn't have selected a more challenging day to sell shares. On Thursday, when the initial public offering was priced at $21 a share, or $8.2 billion, the S&P 500 Index skidded 2.5 percent, the biggest one-day drop in six weeks, and the Dow Jones Industrial Average surrendered 700 points. The…