Revert "Merge branch 'master' into master"

This reverts commit e3a03c04, reversing
changes made to 64749826.

Changed implementation to incorpoarate new init feature for goldctl. Reduces redundant work as well.

When running git update-packages, if the goldens repo is dirty, the
flutter tool currently crashes and logs the failure from git pull.

This adds a check for a dirty git client and emits a friendlier error
message in the case where it's not clean, and avoid the crash by
catching the NonZeroExitCode exception and rethrowing as toolExit
instead.

Fixes: https://github.com/flutter/flutter/issues/28915

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

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.