Fine. Use separate declaredXyz in InstanceItem.

This simplifies the implementation, even if makes it less cool.

Change-Id: I9418f959ca36455a1fff59cd4f980b319351d987
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428562
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: scheglov