nnbd tidying

[pq]

More tidying up.

On balance I think this makes the codebase a lot better and surely more safe. There were a number of places where I was tempted to do deeper structural re-organization but I opted not to in the interest of keeping this more manageable. At some point I do think a full audit would be in order.

There are a few test utilities left after this but this should get lib under control.

Thanks in advance for muddling through!

/cc @bwilkerson

[coveralls]

Coverage decreased (-0.01%) to 96.603% when pulling 8099720 on nnbd_tidy into 7849ee5 on master.

[parlough]

Something that is not super consistent with these changes and elsewhere in null safety migrations is the difference between code similar(but not limited) to the following:

var person = group.leader;
return person != null && person.isCool;

versus

var person = group.leader;
return person?.isCool == true;

I'm not sure which I prefer, but I wonder if there should be a suggestion as part of effective dart for this situation and if we should/could add a potential lint. I'm assuming the preference would be the first one, as while it is not as concise, it is a bit more clear in intentions. Not super related to this PR, but thought I should inquire about it. Perhaps @munificent has some opinions around it.

[bwilkerson]

None of my comments are required, so take them or leave them.

[pq]

@parlough: thanks for noticing this. I almost wrote a note about it. I admit I feel pretty uneasy about code like foo?.isBar == true but sometimes the alternative is even more cumbersome. I'm not sure if there's a once-size-fits-all solution but would be happy if there were one if only so I didn't have to hem and haw each time.

[a14n]

return person?.isCool == true;

The recommendation from Effective dart is:

return person?.isCool ?? false;

Do we have a lint for that?

[pq]

@a14n: I had missed that entirely. I appreciate the rationale too. We should absolutely lint for it though IMO.

dart-lang/sdk#58324