A Waltham-based electric car battery supplier -- now facing financial implosion despite receiving $249 million in federal stimulus cash -- was a heavy donor to congressional Democrats before scoring the hefty taxpayer handout, the Herald has learned.A123 Systems CEO David Vieau has donated $16,900 to Washington, D.C., power brokers and Democratic committees since 2008, including $2,400 to Bay State Rep. Edward J. Markey, the chairman of the climate and energy committees, in 2009 -- just three months before A123 received $249 million in federal stimulus funds. Vieau donated another $1,500 to Markey last year as the company pushed for even more federal dollars through government loans.