[fix] ResponderEventPlugin injection for tree-shaking bundler

Previously this depended on a side-effecting `import` statement which
would be removed under tree-shaking.

packages/react-native-web/src/exports/createElement/index.js

@@ -7,12 +7,18 @@
  * @noflow
  */
 
-import '../../modules/injectResponderEventPlugin';
-
 import AccessibilityUtil from '../../modules/AccessibilityUtil';
 import createDOMProps from '../../modules/createDOMProps';
 import normalizeNativeEvent from '../../modules/normalizeNativeEvent';
 import React from 'react';
+import ReactDOM from 'react-dom';
+import ResponderEventPlugin from '../../modules/ResponderEventPlugin';
+
+const { EventPluginHub } = ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;
+
+EventPluginHub.injection.injectEventPluginsByName({
+  ResponderEventPlugin
+});
 
 /**
  * Ensure event handlers receive an event of the expected shape. The 'button'

packages/react-native-web/src/modules/ResponderEventPlugin/index.js

@@ -1,10 +1,8 @@
 // based on https://github.com/facebook/react/pull/4303/files
 
 import normalizeNativeEvent from '../normalizeNativeEvent';
-import ReactDOM from 'react-dom';
 import ReactDOMUnstableNativeDependencies from 'react-dom/unstable-native-dependencies';
 
-const { EventPluginHub } = ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;
 const { ResponderEventPlugin, ResponderTouchHistoryStore } = ReactDOMUnstableNativeDependencies;
 
 const topMouseDown = 'topMouseDown';
@@ -78,6 +76,4 @@ ResponderEventPlugin.extractEvents = (topLevelType, targetInst, nativeEvent, nat
   );
 };
 
-EventPluginHub.injection.injectEventPluginsByName({
+export default ResponderEventPlugin;
-  ResponderEventPlugin
-});