Add gclient sync to package_config.json presubmit message

Change-Id: I1f53911efc0e809e23bdee4cd522aff9b76c8c53
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/184640
Auto-Submit: Leaf Petersen <leafp@google.com>
Commit-Queue: Leaf Petersen <leafp@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>
Reviewed-by: Gary Roumanis <grouma@google.com>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index ef388b7..7687b1d 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -236,7 +236,8 @@
             output_api.PresubmitError(
                 'File .dart_tool/package_config.json is out of date.\n'
                 'Fix these issues with:\n'
-                '%s tools/generate_package_config.dart' % (dart))
+                'gclient sync -D && %s tools/generate_package_config.dart' %
+                (dart))
         ]
     return []