[Edit eernstg May 20: Updated links.]
The analyzer does not support external fields yet.
The CFE does as of last week (383a8ea).
Language proposal: dart-lang/language#969, tests: https://dart-review.googlesource.com/c/sdk/+/148324.
Creating an issue to point to and track progress.
This work still needs to be prioritized and planned., but the proposal itself is taking shape: cf. the feature spec in dart-lang/language#969 and the tests in https://dart-review.googlesource.com/c/sdk/+/148324.