[camera] Stable release for null safety.

[mvanbeusekom]

Stable NNBD release for the camera package.

With this PR I also migrated the example App to sound null safe. Note that I didn't update the dev-dependency on mockito: ^5.0.0-nullsafety.7 since the stable version causes a conflict with the current flutter_driver dependency. I have marked the dependency in the pubspec.yaml with a # TODO(mvanbeusekom) comment to remind me to remove it when the conflict is resolved.

Conflict details:

Because mockito >=5.0.0 depends on analyzer ^1.0.0 which depends on crypto ^3.0.0, mockito >=5.0.0 requires crypto ^3.0.0.
And because every version of flutter_driver from sdk depends on crypto 2.1.5, mockito >=5.0.0 is incompatible with flutter_driver from sdk.
So, because camera depends on both flutter_driver any from sdk and mockito ^5.0.0, version solving failed.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[stuartmorgan]

I think since this is the last plugin to be published as stable, this will work. Let's see how our bots handle it.

(Ideally we can now sweep the repo to make everything require 2.0.0 instead of a range that includes a non-nullable version.)

[mvanbeusekom]

@stuartmorgan thanks for the feedback. I have gone through all your comments and made the necessary adjustments.

Could you have another look please?

[stuartmorgan]

A few more nits, but otherwise looks good.

[stuartmorgan]

Was this to resolve a conflict? We don't usually use any unless we have to; I would expect 1.16.0 (or 1.16.3 if you need Fake).

[mvanbeusekom]

This was actually a mistake on my end. I added this dependency because I was receiving a CI failure with the dependencies related to the integration_test in combination with running in NNBD. I started comparing with the video_player plugin and noticed that this dependency was included this way and decided to give that a try. Reading back in the Discord channel I realised that we shouldn't run integration_tests in NNBD mode (and add the // @dart = 2.9 instruction to the top of the file). Unfortunately I forgot to remove this dependency which I did this time ;).

[stuartmorgan]

LGTM!