Fix a discovery generation bug where fields that are types with illegal class names cause broken generated code

Previously, where a schema object's name was a reserved keyword, the marshaller's helper method for it was declared as '_handle' but referenced as '_handle$'. This change makes both use '_handle'.

Clone of https://github.com/google/streamy-dart/pull/240 with commit history squashed.

FYI post-rebase it appears that test/generated/schema_object_client_objects.dart is broken - since it's broken in a clean client too I don't believe it's this change that's broken it.