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];
```

'start',
        5
      )).toBe('start="5"');
      expect(DOMPropertyOperations.createMarkupForProperty(
        'start',
        0
      )).toBe('start="0"');
      expect(DOMPropertyOperations.createMarkupForProperty(
        'size',
        0
      )).toBe('');
      expect(DOMPropertyOperations.createMarkupForProperty(
        'size',
        1
      )).toBe('size="1"');
    });
  });
  describe('createMarkupForProperty', function() {
    it('should allow custom properties on web components', function() {
      expect(DOMPropertyOperations.createMarkupForCustomAttribute(
        'awesomeness',
        5
      )).toBe('awesomeness="5"');
      expect(DOMPropertyOperations.createMarkupForCustomAttribute(
        'dev',
        'jim'
      )).toBe('dev="jim"');
    });
  });
  describe('setValueForProperty', function() {