Don't call this method more than once, and only after you've generated all docs for the Package.
Source
void verifyLinks(Package package, String origin) {
assert(_hrefs == null);
_hrefs = package.allHrefs;
final Set<String> visited = new Set();
final String start = 'index.html';
stdout.write('\nvalidating docs...');
_doCheck(package, origin, visited, start);
_doOrphanCheck(package, origin, visited);
_doSearchIndexCheck(package, origin, visited);
}