Backwards compat fix for ReactCurrentDispatcher on older react versions (#14770)

* Add current owner ref fallback for newer renderers with older react versions
* Replaced current owner forward with current:null

Author: bvaughn

packages/shared/ReactSharedInternals.js

@@ -10,4 +10,13 @@ import React from 'react';
 const ReactSharedInternals =
   React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;
 
+// Prevent newer renderers from RTE when used with older react package versions.
+// Current owner and dispatcher used to share the same ref,
+// but PR #14548 split them out to better support the react-debug-tools package.
+if (!ReactSharedInternals.hasOwnProperty('ReactCurrentDispatcher')) {
+  ReactSharedInternals.ReactCurrentDispatcher = {
+    current: null,
+  };
+}
+
 export default ReactSharedInternals;