run dart docs on analysis task (#24172)

dev/bots/analyze.dart

@@ -230,7 +230,7 @@ Future<EvalResult> _evalCommand(String executable, List<String> arguments, {
 Future<void> _runFlutterAnalyze(String workingDirectory, {
   List<String> options = const <String>[]
 }) {
-  return runCommand(flutter, <String>['analyze']..addAll(options),
+  return runCommand(flutter, <String>['analyze', '--dartdocs']..addAll(options),
     workingDirectory: workingDirectory,
   );
 }