Currently all PRs are failing on Travis, but master and presubmit jobs are unaffected. This is due to an unknown issue with pub serve, which is how PRs are tested as opposed to dart2js compilation that happens on presubmit and master branches.

I'm submitting a PR to run dart2js for all PRs until this issue can be investigated. This will add about 10m to each PR, but it's better than outright failing.