Fallback to event.srcElement for IE9

It looks like we accidentally removed a fallback condition for the
event target in IE9 when we dropped some support for IE8. This commit
adds the event target specific support code back to getEventTarget.js

Fixes #12506

packages/react-dom/src/events/getEventTarget.js

@@ -15,7 +15,9 @@ import {TEXT_NODE} from '../shared/HTMLNodeType';
  * @return {DOMEventTarget} Target node.
  */
 function getEventTarget(nativeEvent) {
-  let target = nativeEvent.target || window;
+  // Fallback to nativeEvent.srcElement for IE9
+  // https://github.com/facebook/react/issues/12506
+  let target = nativeEvent.target || nativeEvent.srcElement || window;
 
   // Normalize SVG <use> element events #4963
   if (target.correspondingUseElement) {