Parts with imports

Tracking issue for the language feature, see spec here https://github.com/dart-lang/language/blob/main/working/parts-with-imports/feature-specification.md

Start with working/parts-with-imports/feature-specification.md, linked from the issue, to understand the proposed language feature. This is a tracking issue and does not name implementation files, tests, or acceptance criteria, so the issue alone does not define what completion looks like.

Written by the indexing model from the issue text.