Fix build for pub after new analyzer_experimental snapshot.
Fix build for pub after new analyzer_experimental snapshot.

R=brianwilkerson@google.com, jmesserly@google.com
BUG=

Review URL: https://codereview.chromium.org//23769002

git-svn-id: dae132d4ca6f88932ad8a9ec3641c81ad1592b72@26883 260f80e4-7a28-3924-810f-c04153c831b5

dart-archive/observe,dart-lang/observe