Deprecate legacyImplementation

Summary: The legacy implementation doesn't provide additional functionality and has a negative impact on performance and user experience. The legacyImplementation prop is deprecated and will be removed in a future release.

Reviewed By: yungsters

Differential Revision: D10212762

fbshipit-source-id: 9b3416434ba392827b538c984c7ab4bcbe156e60

Author: elicwhite

Libraries/Lists/FlatList.js

@@ -525,8 +525,9 @@ class FlatList<ItemT> extends React.PureComponent<Props<ItemT>, void> {
       // comparison.
       if (!this._hasWarnedLegacy) {
         console.warn(
-          'FlatList: Using legacyImplementation - some features not supported and performance ' +
-            'may suffer',
+          'FlatList: legacyImplementation is deprecated and will be removed in a ' +
+            'future release - some features not supported and performance may suffer. ' +
+            'Please migrate to the default implementation.',
         );
         this._hasWarnedLegacy = true;
       }