Bring imports along when copy/pasting code

[DanTup]

It's possible the analysis server will gain the ability to bring imports along when copy/pasting:

dart-lang/sdk@d31ed86

We need to be able to hook these events in Code (microsoft/vscode#30066) for this to work, but it'd make a great feature!

We'd need to know the analysis server version it first ships in and only try to use it if it's available. We may also wish to hide it behind a flag while it's marked experimental (probably we'll need to have both experimental + stable version numbers so we can stop requiring the flag for the correct version when it becomes "stable").

[DanTup]

Dupe of #351