Fix stale usage of emptyObject in cloneWithProps-test.

After we run `require('mock-modules').dumpCache()`, the object exported by
the `emptyObject` module will no longer be identical to previously
exported objects, so tests like `expect(component.refs).toBe(emptyObject)`
will fail.

Note that this behavior only manifests itself in tests, because of course
we do not call `dumpCache` in production code.

We could consider storing the `emptyObject` globally to thwart the effects
of `dumpCache`, but it's more idiomatic simply to re-`require` the latest
version of `emptyObject`.

src/utils/__tests__/cloneWithProps-test.js

@@ -24,12 +24,12 @@ require('mock-modules').dontMock('cloneWithProps');
 var mocks = require('mocks');
 
 var cloneWithProps = require('cloneWithProps');
-var emptyObject = require('emptyObject');
 
 var React;
 var ReactTestUtils;
 
 var onlyChild;
+var emptyObject;
 
 describe('cloneWithProps', function() {
 
@@ -37,6 +37,7 @@ describe('cloneWithProps', function() {
     React = require('React');
     ReactTestUtils = require('ReactTestUtils');
     onlyChild = require('onlyChild');
+    emptyObject = require('emptyObject');
   });
 
   it('should clone a DOM component with new props', function() {