[shared_preferences] Fix initialization race

[stuartmorgan-g]

During the NNBD transition, the structure of the completer logic in the initialization flow was incorrectly changed to set the field after an await instead of immediately.

Also updates the error handling to handle Error the same way it currently handles Exception, which this change surfaced.

Fixes flutter/flutter#42407

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[tarrinneal]

Interesting. The setPrefix method is getting an unimplemented error on some tests now.

[stuartmorgan-g]

Interesting. The setPrefix method is getting an unimplemented error on some tests now.

I missed that only half of the tests were in the group that sets an instance during setUp (making my change to instance affect other tests). Fixing now.

[stuartmorgan-g]

GitHub isn't showing my push here yet since it's having some kind of semi-outage at the moment, but it's up at main...stuartmorgan:packages:shared-prefs-init

The test change looks enormous, but I just removed the generic group that most-but-not-all of the tests were in, so now they are all getting the same setup. (I'm not sure why the pattern in so many of our plugins is that there's a group containing all of the tests.)

[tarrinneal]

I'm surprised this worked at all before.

[stuartmorgan-g]

I'm surprised this worked at all before.

Well, to hit this race you have to call getInstance again between the very first time in the app that you call getInstance and the initial _getSharedPreferencesMap call completing, which is likely to be a very small window of time. And then for it to be an actual problem, you have to either keep both of those instances around long term, or you have to keep (the wrong) one of them long term, but then also sometimes make new getInstance calls.

I can't image that's a common use pattern.