attempt to provide a working CustomEvent jest mock so that tests can run

Author: JoshuaGross

packages/react-native-renderer/src/ReactNativeBridgeEventPlugin.js

@@ -199,6 +199,7 @@ const ReactNativeBridgeEventPlugin = {
     const bubbleDispatchConfig = customBubblingEventTypes[topLevelType];
     const directDispatchConfig = customDirectEventTypes[topLevelType];
     let customEventConfig = null;
+    console.log('what is CustomEvent?', CustomEvent);
     if (nativeEvent instanceof CustomEvent) {
       // $FlowFixMe
       if (topLevelType.indexOf('on') !== 0) {

packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/CustomEvent.js

@@ -0,0 +1,14 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+// See the react-native repository for a full implementation.
+// This is just a stub, currently to pass `instanceof` checks.
+const CustomEvent = jest.fn();
+
+module.exports = {default: CustomEvent};