You are correct that the quality of packages can vary in Pub or any other pack repo. Here are a few things you could use to evaluate the quality of the packages:
- Is the package actively maintained?
- How many active committers does it have?
- How many people have starred or forked it on GitHub?
- How much use do you think it is getting? Are there questions about it on StackOverflow or other mailing lists?
To answer your specific questions:
- You can reasonably expect "Dart team" packages to be supported.
- There isn't a list of official Google supported packages. Just look for packages supported by the Dart team if you're looking for packages created by members of the Dart project.
- The Dart project doesn't currently have any way of ranking Pub packages.