* Exclude flutter_goldens package from dartdoc because it's for internal
use only
* Document why flutter_tools doesn' tneed to be excluded from the list of
packages to document
* Performance optimization in the flutter comparator, and associated
test updates.
In order to avoid checking binaries into flutter/flutter,
this comparator can be used to retrieve golden files from
a sibling flutter/goldens repository.
https://github.com/flutter/flutter/issues/16859
