Dart libraries for both Google APIs and Protobufs are available for download. You can use Dart to access APIs from services such as Google+, YouTube, Drive, and more. You can also use Dart to interact with Protocol Buffers, Google's structured data encoding format. The libraries are open source and available today.
The Dart client libraries for Google APIs cover a wide range of Google APIs. Each client library is bundled as a pub package and uploaded to pub.dartlang.org, Dart's package hosting service. For example, check out the Dart library for the YouTube API on pub. These libraries are maintained by a dedicated group of Dart community developers.
Bonus: learn how to generate Dart client libraries for your own Cloud Endpoint service in this post from the Dartwatch blog.
Protocol Buffers, or protobufs, are a way to encode and decode structured data for interop between various systems. Both the protobuf runtime and code generator are available for Dart, and are open source.