Replace wrappedElement with firstFragment.

`wrappedElement` was being used to represent local elements before
`firstFragment` existed, but now that it does, it's best to use
consistent naming.

Change-Id: Iee07d56b93fa9201b49c4eb37c97bfd4de11d97a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445603
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: fshcheglov