flutter_tools cannot depend on flutter_goldens (as flutter_goldens
depdends on the Flutter sdk), so this commit splits client.dart from
flutter_goldens to a pure-dart flutter_goldens_client package.
This enables using GoldensClient from the flutter driver file for the
embedded Android views test.
flutter_goldens.dart has a transitive dependency on dart:ui so it cannot
be included in non Flutter apps. Splitting it out allows us to use it
from driver tests.
This also removes the visibleForTesting annotation from GoldensClient.
* 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
