We can replace uses of the @checked annotation with real syntax (replace @checked with covariant), once this rolls into your version of the SDK. The @checked annotation was the prototype, this is the real thing.

See dart-lang/sdk#27486

For an example, see https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#1220