Roll of https://github.com/dart-lang/native/pull/964, which separates the `KernelAsset`s (the asset information embedded in the Dart kernel snapshot) from `Asset`s (the assets in the `build.dart` protocol). See the linked issue for why they ought to be different instead of shared.

This PR does not change any functionality in Flutter.

(Now that https://github.com/flutter/flutter/pull/143055 has landed, we can land breaking changes.)

For reference, the same roll in the Dart SDK: https://dart-review.googlesource.com/c/sdk/+/352642