anchor.diff:              'Use <base href> in generated files (legacy behavior). This option '
               'is temporary and support will be removed in the future. Use only '
               'if the default behavior breaks links between your documentation '
  -            'pages, and please file an issue on Github.',
  +            'pages, and please file an issue on GitHub.',
           negatable: false,
           hide: true),
       // TODO(jdkoren): Unhide when we have good support for another format.
  
positive.code:     }
    }
  
    /// Try to find an explicit argument setting this value, but if not, fall back to files
    /// finally, the default.
    @override
    T valueAt(Directory dir) {
      var value = _valueAtFromArgs();
  
negative.code:       example = '0.76';
      }
      throw DartdocOptionError(
          'Invalid argument value: --${argName}, set to "${value}", must be a '
          '${T}.  Example:  --${argName} ${example}');
    }
  
    /// Returns null if no argument was given on the command line.
  