Some (all?) windows bots got stuck into an infinite loop trying to download the Dart SDK.

Examples:

• https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8724395172445781505/+/u/flutter_config_--clear-features/stdout
• https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8724395172445781489/+/u/flutter_config_--clear-features/stdout

The scripts need to be fixed such that they have a maximum number of retries.

cc @christopherfujino