FetchProgress.connecting constructor
Fetch is connecting
Implementation
@visibleForTesting
/// Fetch is connecting
FetchProgress.connecting(this.fetch, {required this.prior})
    : progress = 0.0,
      length = 0,
      downloaded = 0,
      status = FetchStatus.connecting,
      headers = null,
      responseCode = null {
  prior?.prior = null;
}