mergedResults.sent is never used. It doesn't appear to be used when it was introduced in D7815

The additional of Error | here is explained below

Rewriting this as a map let me skip annotating some types

This is necessary because of the Error | change mentioned above, which is explained below

Since Flow no longer believes that error is necessarily a FirebaseError, I have to add all of these runtime checks to confirm error.errorInfo.errorCode is what I think it is

This is the reason for Error |. The exception caught here can be returned in the errors array.

(The truth is that you can throw anything in JS, not just an Error. But throwing Error is a convention.)

Flow wanted me to pull this out

Not sure when or why this was necessary, but there's no reason it would be risky to return result directly – it's constructed right above on line 340 and not passed to anyplace else

mergedResults.sent is never used. It doesn't appear to be used when it was introduced in D7815

The addition of | mixed here is explained below

Rewriting this as a map let me skip annotating some types

This is necessary because of the | mixed change mentioned above, which is explained below

Since Flow no longer believes that error is necessarily a FirebaseError, I have to add all of these runtime checks to confirm error.errorInfo.errorCode is what I think it is

This is the reason for | mixed. The exception caught here can be returned in the errors array.

Flow wanted me to pull this out

Not sure when or why this was necessary, but there's no reason it would be risky to return result directly – it's constructed right above on line 340 and not passed to anyplace else

I hear you... it's more natural to me too. I initially opted for "Writable" as a contrast to Flow's $ReadOnly, though – I think read/write matches better than read/mutable. At this point it would take a lot of time to rename all of the Writable* types I've created

That's fair. I'll take a look at doing that (without changing the format of what we store in MariaDB)

True, but there's no code duplication in the WritableFCMPushResult definition, and this lets us avoid recreating the object on every write