I was looking at https://github.com/facebook/react/issues/12700, and this seemed suspicious.

Consider this case:

```js
class App extends React.Component {
  componentDidMount() {
    for (let i = 0; i < 10000; i++) {
      ReactDOM.render(<div />, document.createElement('div'));
    }   
  }
  render() {
    return null
  }
}

ReactDOM.render(<App />, document.getElementById('root'));
```

Profiling tells me we spend a *lot* of time searching for a root:

<img width="838" alt="screen shot 2018-08-07 at 12 12 52 am" src="https://user-images.githubusercontent.com/810438/43745499-f8716922-99d6-11e8-8822-380dcb15bfa3.png">

Adding some logging shows that e.g. for a 1000 roots, we'll run [this loop iteration](https://github.com/facebook/react/blob/master/packages/react-reconciler/src/ReactFiberScheduler.js#L1798-L1846) 1000 times for the first root, then 999 times for the second root, then 998 times for the third root, etc. Oops.

This is surely a bug?
if (firstBatch !== null && firstBatch._expirationTime <= expirationTime) {
    if (completedBatches === null) {
      completedBatches = [firstBatch];
    } else {
      completedBatches.push(firstBatch);
    }
    if (firstBatch._defer) {
      
      
      root.finishedWork = finishedWork;
      root.expirationTime = NoWork;
      return;
    }
  }
  
  root.finishedWork = null;
  
  
  if (root === lastCommittedRootDuringThisBatch) {
    
    
    nestedUpdateCount++;
  } else {
    
    lastCommittedRootDuringThisBatch = root;
    nestedUpdateCount = 0;
  }
  commitRoot(root, finishedWork);
}
function shouldYield() {