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 (legacyErrorBoundariesThatAlreadyFailed === null) {
    legacyErrorBoundariesThatAlreadyFailed = new Set([instance]);
  } else {
    legacyErrorBoundariesThatAlreadyFailed.add(instance);
  }
}
function commitRoot(root: FiberRoot, finishedWork: Fiber): void {
  isWorking = true;
  isCommitting = true;
  startCommitTimer();
  invariant(
    root.current !== finishedWork,
    'Cannot commit the same tree as before. This is probably a bug ' +
      'related to the return field. This error is likely caused by a bug ' +
      'in React. Please file an issue.',
  );
  const committedExpirationTime = root.pendingCommitExpirationTime;
  invariant(
    committedExpirationTime !== NoWork,
    'Cannot commit an incomplete root. This error is likely caused by a ' +
      'bug in React. Please file an issue.',
  );
  root.pendingCommitExpirationTime = NoWork;
  
  
  
  const updateExpirationTimeBeforeCommit = finishedWork.expirationTime;
  const childExpirationTimeBeforeCommit = finishedWork.childExpirationTime;
  const earliestRemainingTimeBeforeCommit =
    updateExpirationTimeBeforeCommit === NoWork ||