Augment. Skip field augmentations when checking for duplicate members.

Treat field fragments marked as augmentations like other augmented
members when collecting local member names. Do not add their getter and
setter fragments to the instance or static duplicate scopes, because
they augment existing declarations rather than introduce new local
members.

This avoids secondary duplicateDefinition and conflicting member
diagnostics when a field augmentation targets an existing declaration.
Non-augmenting declarations in augmentation blocks are still checked as
new declarations, so real duplicates continue to be reported.

We don't have yet checks for completed getter / setter from fields and
variables, so tests miss several errors.

Change-Id: I6901aea1770891917c966c11b59cb647aaf4ed86
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/503840
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>