Update dartdoc to 0.30.2 (#52022)

dev/bots/docs.sh

@@ -111,7 +111,7 @@ if [[ -d "$FLUTTER_PUB_CACHE" ]]; then
 fi
 
 # Install and activate dartdoc.
-"$PUB" global activate dartdoc 0.29.3
+"$PUB" global activate dartdoc 0.30.2
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.

dev/tools/dartdoc.dart

@@ -136,6 +136,7 @@ Future<void> main(List<String> arguments) async {
     '--link-to-source-root', '../..',
     '--link-to-source-uri-template', 'https://github.com/flutter/flutter/blob/master/%f%#L%l%',
     '--inject-html',
+    '--use-base-href',
     '--header', 'styles.html',
     '--header', 'analytics.html',
     '--header', 'survey.html',