Support auto-importing from code completion #1060
Description
Requires some work in the server first:
- Analysis Server completion results should include info on where to insert imports dart-lang/sdk#27220
- Could I code complete from a library that I've imported, even if it's not in my "show" list? dart-lang/sdk#25298
- Automatically insert import on code completion dart-lang/sdk#21931
- suggest identifiers that are not yet imported dart-lang/sdk#25820