Fine. Use BaseNameMembers.

So, we can use types to define some combinations out of existence.

But it also supports valid combinations better than the previous approach with a map from `LookupName` to a member or constructor.

Change-Id: If06ad1e9636a994d6164eb24d5cec163061a4138
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/424081
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov