fix nits from recent polymer roll

Fixed two nits from the recent polymer roll (https://github.com/dart-lang/spark/pull/1272):

- an analyzer class was renamed for the `analyzer-0.13.0-dev.5` version
- fix a missing css error message from running Spark in Dartium. I've verified that the new css reference works for both Dartium and the deployed dart2js version.

@ussuri