An unprecedented wave of bad news has swamped this sector over the past year, with firm after firm reporting multibillion-­dollar credit-­related ­losses stemming from the mortgage market collapse, the demise of Bear Stearns Cos. last spring and its government-­­backed take­over — at $10 a share — by JPMorgan Chase & Co. in May and, last month, the bankruptcy of Lehman Brothers Holdings and the subsequent sale of its investment banking unit to Barclays, the agreed sale of Merrill Lynch & Co. to Bank of America Corp. and the conversion of Goldman, Sachs & Co. and Morgan Stanley to bank holding companies.