The Dart test package is not part of the SDK itself, so it has to be installed as a normal third-party dependency. You should already know how to do this.
For reference, check the example, 4_unit_tests, in this chapter's source code on GitHub. The test code is located inside the test/ folder.
In this example (generated with the Stagehand tool), there is a development dependency; this is a dependency that is required only during development and not at runtime:
dev_dependencies:
test: ^1.0.0
This enables us to use the test package's provided libraries to write unit tests.