Copy Differ implementation to new file, feature-flag-gate new differ

Summary:
Changes in following diffs will be gated by this feature flag.

The differ in the new file is copied from the current stable implementation and will not be modified until it's deleted.

Changelog: [Internal]

Reviewed By: sammy-SC, mdvacca

Differential Revision: D27775698

fbshipit-source-id: 03d9518ffd2b1f25712386c56a38bd2b4d839fc2

ReactCommon/react/renderer/mounting/Differentiator.cpp

@@ -6,6 +6,7 @@
  */
 
 #include "Differentiator.h"
+#include "DifferentiatorFlatteningClassic.h"
 
 #include <better/map.h>
 #include <better/small_vector.h>
@@ -1583,7 +1584,13 @@ ShadowViewNodePair::List sliceChildShadowNodeViewPairsLegacy(
 
 ShadowViewMutation::List calculateShadowViewMutations(
     ShadowNode const &oldRootShadowNode,
-    ShadowNode const &newRootShadowNode) {
+    ShadowNode const &newRootShadowNode,
+    bool useNewDiffer) {
+  if (!useNewDiffer) {
+    return DifferOld::calculateShadowViewMutations(
+        oldRootShadowNode, newRootShadowNode);
+  }
+
   SystraceSection s("calculateShadowViewMutations");
 
   // Root shadow nodes must be belong the same family.

ReactCommon/react/renderer/mounting/Differentiator.h

@@ -23,7 +23,8 @@ enum class ReparentMode { Flatten, Unflatten };
  */
 ShadowViewMutationList calculateShadowViewMutations(
     ShadowNode const &oldRootShadowNode,
-    ShadowNode const &newRootShadowNode);
+    ShadowNode const &newRootShadowNode,
+    bool useNewDiffer);
 
 /**
  * Generates a list of `ShadowViewNodePair`s that represents a layer of a