Elements. Replace Member._element2 with baseElement.

Change-Id: I7dff2adac95ed8daf779e2474a3a2b11d6bde584
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/438302
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov