Most of Google's external APIs are described in JSON document by an API called the Discovery Service. The document is called the discovery document and provides enough information to generate a client API that can represent the APIs resources as objects and can connect over HTTP to make calls. The Dart libraries you linked to were generated this way.
You can read about the Discovery Service here: https://developers.google.com/discovery/v1/using
And the generator that those libraries used is here: https://github.com/dart-gde/discovery_api_dart_client_generator