Summary:
After fixing `calculateShadowViewMutationsForNewTree` I realized that it will be even better to test Stacking Context and mutation instructions infra using both functions: `calculateShadowViewMutationsForNewTree` (used for testing) and the Differentiator itself. This diff implements it.
Now we have two similarly working functions with different implementations that we can use for testing Differentiator and other parts of the infra.
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D25576922
fbshipit-source-id: 7922e9ebfb9d6ef1792566554ba0c4a14f835ae2