Stopgap generating docgen all at once until --append mode can be fixed

pkg/docgen/bin/docgen.dart

Index: pkg/docgen/bin/docgen.dart
diff --git a/pkg/docgen/bin/docgen.dart b/pkg/docgen/bin/docgen.dart
index 76cb9d45900d1f6ba8a784340ef0006e156b88c6..cb045378ef144b675a28fd6604519e6fee4aa8f7 100644
--- a/pkg/docgen/bin/docgen.dart
+++ b/pkg/docgen/bin/docgen.dart
@@ -25,7 +25,7 @@ void main(List<String> arguments) {
       includeSdk: results['parse-sdk'] || results['include-sdk'],
       parseSdk: results['parse-sdk'],
       append: results['append'] && new Directory('docs').existsSync(),
-      introduction: results['parse-sdk'] ?
+      introduction: (results['parse-sdk'] || results['include-sdk']) ?
           'sdk-introduction.md' : results['introduction']);
 }