We are currently in the middle of a migration process. This package is still developed under the Dart repo in branches/bleeding_edge/dart/pkg/web_components/. Until the migration is complete, we are not accepting pull requests on this repo.
Even for small changes, we ask that you please sign the CLA electronically here. The CLA is necessary because you own the copyright to your changes, even after your contribution becomes part of our codebase, so we need your permission to use and distribute your code. You can find more details here.