dart2js: Revert r36565 (Work-around for memory leak).

After fixing VM bug dartbug.com/18886 this work-around is not
needed anymore.

I tested dart2js tests in batch mode using the following command.

tools/test.py -mrelease -aia32 -cdart2js -rd8 --dart2js-batch

and it works fine.

Review URL: https://codereview.chromium.org//678083003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41434 260f80e4-7a28-3924-810f-c04153c831b5