Store splices directly on array when legacyUndefined is set

Legacy behavior stored splices non-ephemerally. To most easily match this behavior, we store splices directly on the array. This was previously avoided because the property will show as an enumerable property on the array; however, this avoids the need to store and clear splices in a more bespoke way.

lib/mixins/property-effects.js

@@ -1008,8 +1008,18 @@ function getArgValue(data, props, path) {
  * @private
  */
 function notifySplices(inst, array, path, splices) {
-  inst.notifyPath(path + '.splices', { indexSplices: splices });
+  const splicesData = { indexSplices: splices };
+  // Legacy behavior stored splices in `__data__` so it was *not* ephemeral.
+  // To match this behavior, we store splices directly on the array.
+  if (legacyUndefined) {
+    array.splices = splicesData;
+  }
+  inst.notifyPath(path + '.splices', splicesData);
   inst.notifyPath(path + '.length', array.length);
+  // Clear splice data only when it's stored on the array.
+  if (legacyUndefined) {
+    splicesData.indexSplices = [];
+  }
 }
 
 /**