Add `petitparser` (#906)

See: https://github.com/dart-lang/dart-pad/issues/2101
diff --git a/lib/src/project.dart b/lib/src/project.dart
index 144d0d4..ec19d66 100644
--- a/lib/src/project.dart
+++ b/lib/src/project.dart
@@ -105,7 +105,8 @@
         'animations',
         'equatable',
         'matcher',
-        'flutter_map'
+        'flutter_map',
+        'petitparser',
       ],
     };