[dart2js] size_estimator: take care with partially finalized names

ModularNames can be 'finalized' to Namer Names that are not
finalized. This matters only when using the size-estimator to inform
partitioning, which scans the JS ASTs before finalizing names related
to the structure of deferred-loaded parts (i.e. 'holders').

A better fix would be to have a clearer distinction between shallow
finalization (perhaps 'resolution') and deep finalization, but this
fix is less risky if we decide to cherry-pick.

Bug: #64150
Change-Id: Ib1f9be92f90764389c36b0a7ccf14295bf062490
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/543120
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>