Fix ES6Class test so it runs in browsers

document.createElement throws an error if you don't pass it any
arguments. Neither PhantomJS nor jest (via jsdom at version jest is
using) behave as browsers do now so this went unnoticed.

src/modern/class/__tests__/ReactES6Class-test.js

@@ -22,7 +22,7 @@ describe('ReactES6Class', function() {
 
   beforeEach(function() {
     React = require('React');
-    container = document.createElement();
+    container = document.createElement('div');
     attachedListener = null;
     renderedName = null;
     Inner = class extends React.Component {