Original: invertase/dart_firebase_apis#7
To put it simply, this document:

produces the following error in the conversion:

I can reproduce and more than willing to hop on a call to test it.
Im not versed in code generation so I'd appreceate someone stepping in with a PR.
Original Issue in Firebase Admin SDK:
invertase/dart_firebase_admin#29
See also Firestore official docs mentioning NaN values:
https://firebase.google.com/docs/firestore/manage-data/data-types