Make a preview release for Dart 2.12. beta

[dcharkes]

We should make a preview release for the Dart 2.12 beta, in anticipation of the Dart 2.12 stable release.

The version number of this package does not need a suffix anymore.

See 'preview' release on package:ffi for example. https://pub.dev/packages/ffi/versions

[mannprerak2]

We can do a preview release after merging dart-archive/ffigen#147 and dart-archive/ffigen#148 (where we upgrade all dependencies).
package:quiver however, still uses a suffix in its latest null safe version. Should we wait for it to make a preview release first?

[dcharkes]

https://pub.dev/packages/ffigen/versions/2.0.0

[dcharkes]

https://pub.dev/packages/quiver/versions/3.0.0, we can release a 2.0.1.