Dart has a large collection of built in packages. You can read about the core packages at:
However, sometimes you need a specialised package.
There are thousands of third party packages that you can use in your DCli scripts which can be found at:
NOTE: you can't use Flutter or web packages in your DCli scripts.
To use an external package you need to add it as a dependency to your script.
Dart's dependency management is done via a pubspec.yaml file.
Each package includes install instructions which is simply a matter of adding a dependency line to your pubspec and running: