_Temporarily_ retry failing browser tests on Safari, because versions 6.2 and 7.1 are flaky on dart2js output.

Add support for macos 10.9 to buildbot scripts.

BUG=dartbug.com/21434
R=ricow@google.com

Review URL: https://codereview.chromium.org//687213002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41394 260f80e4-7a28-3924-810f-c04153c831b5