Don't include warning in production (#10946)

Fixes a size regression I introduced in #10802.

Author: gaearon

src/renderers/dom/shared/eventPlugins/SimpleEventPlugin.js

@@ -24,7 +24,10 @@ var SyntheticUIEvent = require('SyntheticUIEvent');
 var SyntheticWheelEvent = require('SyntheticWheelEvent');
 
 var getEventCharCode = require('getEventCharCode');
-var warning = require('fbjs/lib/warning');
+
+if (__DEV__) {
+  var warning = require('fbjs/lib/warning');
+}
 
 import type {TopLevelTypes} from 'BrowserEventConstants';
 import type {