Currently, you can add doc imports to any library whether or not it's used in any references.

/// @docImport 'libb.dart'; // unused and has no warning
library a;

/// Test doc comment
class A {}

It would be nice to have it report UNUSED_IMPORT or UNUSED_DOC_IMPORT when the doc import isn't being used for any reference resolving so we can avoid unnecessary doc imports (and also see that it will actively be used).

cc. @srawlins @goderbauer