anchor.diff:    void setOptimizationLevel(String argument) {
       int value = int.tryParse(extractParameter(argument));
       if (value == null || value < 0 || value > 4) {
  -      helpAndFail("Error: Unsupported optimization level '$argument', "
  +      helpAndFail("Unsupported optimization level '$argument', "
             "supported levels are: 0, 1, 2, 3, 4");
         return;
       }
  
positive.code:       passThrough(argument);
        return;
      }
      helpAndFail("Error: Unsupported dump-info format '$argument', "
          "supported formats are: json or binary");
    }
  
    String nullSafetyMode = null;
    void setNullSafetyMode(String argument) {
      if (nullSafetyMode != null && nullSafetyMode != argument) {
        helpAndFail("Error: cannot specify both $nullSafetyMode and $argument.");
      }
      nullSafetyMode = argument;
      passThrough(argument);
  
negative.code: 
    void setOutput(Iterator<String> arguments) {
      outputSpecified = true;
      String option = arguments.current;
      String path;
      if (option == '-o' || option == '--out' || option == '--output') {
        if (!arguments.moveNext()) {
          helpAndFail("Missing file after '$option' option.");
        }
        path = arguments.current;
      } else {
        path = extractParameter(option);
      }
      out = Uri.base.resolve(fe.nativeToUriPath(path));
    }
  