[analyzer] Support external fields.

[dcharkes]

[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.

cc @stereotype441

[eernstg]

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.

[stereotype441]

As of 3384992, the analyzer fully supports abstract fields and external fields/variables.