Unsupported enum values

[via-guy]

Fixes #838.

If an enum value is nullable then an unsupported value should be returned as null, to avoid crashing the parser if a new value is introduced on the repository side.

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[guykogus]

@googlebot I consent

[jeiea]

I'm awaiting this PR to be merged but it stays over 2 month.
Is there anyone to comment for required next steps for being merged or reason of not being merged?

[kevmoo]

@via-guy – are you able to rebase this? Does someone else want to take this over?

[via-guy]

@kevmoo do you need me to rebase? You can just squash and merge, no?

[via-guy]

Ah, or do you mean for post-nnbd?

[via-guy]

Ah, or do you mean for post-nnbd?

I've created #917 for that