Also, add separate 'get library'.
Medium term we will switch to `enclosingElement3` that will return
the unit element that includes this unit element (or null for defining).
Long term, with the new model, these will be fragments.
Change-Id: I999c99df610ad5792dbb3307c55e2b7577afacc6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/377271
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Konstantin Shcheglov
b7d3cdb5a7