anchor.diff:          break;
         case OutputMode.capture:
         case OutputMode.discard:
  -        stdout.writeln(flattenToString(await savedStdout));
  -        stderr.writeln(flattenToString(await savedStderr));
  +        stdout.writeln(_flattenToString(await savedStdout));
  +        stderr.writeln(_flattenToString(await savedStderr));
           break;
       }
       print(
  
positive.code:   }
  }
  
  T identity<T>(T x) => x;
  
  /// Flattens a nested list of UTF-8 code units into a single string.
  String flattenToString(List<List<int>> chunks) =>
    utf8.decode(chunks.expand<int>(identity).toList(growable: false));
  
  /// Specifies what to do with command output from [runCommand].
  enum OutputMode { print, capture, discard }
  
negative.code:   print('$clock ELAPSED TIME: $bold${elapsedTime(start)}$reset for $commandDescription in $relativeWorkingDir: ');
  
    if (output != null) {
      output.stdout = _flattenToString(await savedStdout);
      output.stderr = _flattenToString(await savedStderr);
    }
  
    // If the test is flaky we don't care about the actual exit.
  