title: stub performance.now() using Sinon.js
body: <p>I am writing unit tests in Ember-qunit. I want to set a custom value on performance.now.</p>
  
  <p>I tried <code>sinon.stub(performance,'now', 60000);</code> but this didn't work. I get <code>TypeError: stub(obj, 'meth', fn) has been removed.</code></p>
  
  <p>how do i stub performance.now() using sinon.js?</p>
  
  <p>Thanks</p>