proto3 hazzers always generated

I'm not sure whether this is an issue or intended. In the generated dart code the `has..` method is always generated for a corresponding `message` field regardless of being labeled as `optional` or not. Does it make sense to generate those methods for none-optional fields?

Inspect the generated Dart output for proto3 message fields and compare behavior for optional and non-optional fields. First establish from the generator's existing tests or documented protobuf semantics whether has.. methods are expected for non-optional fields; done means the intended behavior is confirmed and covered consistently.

Written by the indexing model from the issue text.