Replace additionalOptions (#3356)

Author: srawlins

lib/options.dart

@@ -80,15 +80,18 @@ List<DartdocOption<bool>> createDartdocProgramOptions(
 
 DartdocProgramOptionContext? parseOptions(
   PackageMetaProvider packageMetaProvider,
-  List<String> arguments,
-) {
+  List<String> arguments, {
+  // Additional options are given in google3.
+  OptionGenerator? additionalOptions,
+}) {
   var optionRoot = DartdocOptionRoot.fromOptionGenerators(
       'dartdoc',
       [
         createDartdocOptions,
         createDartdocProgramOptions,
         createLoggingOptions,
         createGeneratorOptions,
+        if (additionalOptions != null) additionalOptions,
       ],
       packageMetaProvider);