Call callbacks from setState in component context

This is way more useful than the alternative.

src/core/ReactUpdates.js

@@ -54,7 +54,7 @@ function batchedUpdates(callback) {
         component.performUpdateIfNecessary();
         if (callbacks) {
           for (var j = 0; j < callbacks.length; j++) {
-            callbacks[j]();
+            callbacks[j].call(component);
           }
         }
       }

src/core/__tests__/ReactUpdates-test.js

@@ -237,6 +237,7 @@ describe('ReactUpdates', function() {
     ReactUpdates.batchedUpdates(function() {
       instance.setState({x: 1}, function() {
         instance.setState({x: 2}, function() {
+          expect(this).toBe(instance);
           innerCallbackRun = true;
           expect(instance.state.x).toBe(2);
           expect(updateCount).toBe(2);