update polymer to include the latest barback.
update polymer to include the latest barback.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//89863003

git-svn-id: dae132d4ca6f88932ad8a9ec3641c81ad1592b72@30689 260f80e4-7a28-3924-810f-c04153c831b5

dart-archive/observe,dart-lang/observe