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];
```

ReactPerf.start();
    fn();
    ReactPerf.stop();
    return ReactPerf.getLastMeasurements();
  }
  it('should count no-op update as waste', function() {
    var container = document.createElement('div');
    ReactDOM.render(<App />, container);
    var measurements = measure(() => {
      ReactDOM.render(<App />, container);
    });