Most of dartdoc's tests are large end-to-end tests which read real files in real packages, in the testing/ directory. Unit tests exist in test/unit/.
Many of the end-to-end test cases should be rewritten as unit tests.
At some point, the distinction should flip, such that unit tests are generally located in test/, and end-to-end tests are found in a specific directory, or in files whose names signify the distinction.