Restrict TestOn to library level (#1704)

Closes https://github.com/dart-lang/sdk/issues/49071

Add a `Target` with the `TargetKind.library` so that the analyzer
understands that the annotation is logically applied to the library,
even when it is syntactically applied on an import. This fixes behavior
when sorting imports.
The same configuration was applied to other imports in #1345 and this
looks like it was an accidental omission.
6 files changed