Optionally typed API

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When dart-lang/language#283 lands we will be able to allow optional type parameters

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related to #583

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Aside from language support for optional types, the biggest challenge here is that it introduces a serializer concern to the core library, which then requires some middleware for every unique code generator.

If we can pull it off, I think the most elegant solution would either be a gql link that handles serialization, or adding optional types to the entire gql link system and using its preexisting serialization and parsing.