Change NodeChange.apply to accept a FixAggregator rather than a callback.
Change NodeChange.apply to accept a FixAggregator rather than a callback.

This will facilitate a follow-up CL that introduces a factory class
for creating EditPlans.

Change-Id: I4fa82371844388f3042107375eb1e7a7e9804647
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/129801
Reviewed-by: Mike Fairhurst <2002604e76601be549134505841fea93de80dc45@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk