The fuchsia bucket does not have a delete ACL which is required to replace files that already exist. This change skips uploading the file if it already exists. Bug: https://github.com/flutter/flutter/issues/126461 Bug: https://github.com/flutter/flutter/issues/135189 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style