Plugin dart constraints causes warning during publishing

[cyanglaz]

We start seeing some warning during publishing 1st party plugins.

Package validation found the following potential issue:
* Packages with an SDK constraint on a pre-release of the Dart SDK should themselves be published as a pre-release version. If this package needs Dart version 2.0.0-dev.28.0, consider publishing the package as a pre-release instead.
  See https://dart.dev/tools/pub/publishing#publishing-prereleases For more information on pre-releases.

It hasn't (yet) blocked us from publishing the plugins.

[stuartmorgan-g]

In general we don't need the ability to do this, and we worked around it for the one case where we did (NNBD), so I think it's safe to close this.

[github-actions]

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