I was testing latest refactor of PerfTools.
When using printOperations() often (but not always) I get 

```
Uncaught TypeError: Cannot read property 'displayName' of undefined
```

in getOperations() on the line:

```
var {displayName, ownerID} = treeSnapshot[instanceID];
```

expect(foobarSetter.mock.calls[0][1]).toBe('cows say moo');
    });
    it('should set className to empty string instead of null', function() {
      DOMPropertyOperations.setValueForProperty(
        stubNode,
        'className',
        'selected'
      );
      expect(stubNode.className).toBe('selected');
      DOMPropertyOperations.setValueForProperty(
        stubNode,
        'className',
        null
      );
      
      
      expect(stubNode.className).toBe('');
      expect(stubNode.getAttribute('class')).toBe(null);
    });
    it('should remove property properly for boolean properties', function() {
      DOMPropertyOperations.setValueForProperty(
        stubNode,
        'hidden',
        true
      );
      expect(stubNode.hasAttribute('hidden')).toBe(true);
      DOMPropertyOperations.setValueForProperty(
        stubNode,
        'hidden',
        false