Elements. Switch Member from being FragmentImpl based to 'ElementImpl baseElement'.

Change-Id: I5df8201b5e3d97817a3a1a09683fa7407ff78a46
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/438301
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov