At present, we build dart artifacts in the dist dir during the build, with package overrides for tests. We then leave the dist folder in that state. Later, when we publish to pub, we run pubspec_cleaner.js to get the real files we upload.
Now that we publish dart snapshots for every green travis build, it would be better to do this cleaner during the build. We can run it after tests are finished, so we don't have to do anything fancy to keep them working.