Fix Event Normalization in IE<9

IE<9 relied on the `target` property being overriden. This adds back a hack that only applies in IE<9. I'll be able to revert this hack once I check in synthetic events.

src/core/ReactEventTopLevelCallback.js

@@ -68,6 +68,11 @@ var ReactEventTopLevelCallback = {
       if (!_topLevelListenersEnabled) {
         return;
       }
+      // TODO: Remove when synthetic events are ready, this is for IE<9.
+      if (nativeEvent.srcElement &&
+          nativeEvent.srcElement !== nativeEvent.target) {
+        nativeEvent.target = nativeEvent.srcElement;
+      }
       var topLevelTarget = ReactInstanceHandles.getFirstReactDOM(
         getEventTarget(nativeEvent)
       ) || ExecutionEnvironment.global;

src/core/ReactInstanceHandles.js

@@ -258,7 +258,7 @@ var ReactInstanceHandles = {
    * Traverses up the ancestors of the supplied node to find a node that is a
    * DOM representation of a React component.
    *
-   * @param {?DOMEventTarget} node
+   * @param {*} node
    * @return {?DOMEventTarget}
    * @internal
    */