Support while loop in analyzer2dart.

BUG=
R=paulberry@google.com, sigurdm@google.com

Review URL: https://codereview.chromium.org//686743002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41390 260f80e4-7a28-3924-810f-c04153c831b5