reenable require_trailing_commas since trailing_commas: preserve just landed

[dkbast]

It would be great to reenable require_trailing_commas again once the next version with preserve_trailing included is released.

See:
dart-lang/dart_style#1672
dart-lang/dart_style@71ac085

[spydon]

Since this is out now it would be great to get this into the repo, the only thing that need to be added is this:

formatter:
  trailing_commas: preserve