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?
}
let lastUniqueAsyncExpiration: number = 0;
let expirationContext: ExpirationTime = NoWork;
let isWorking: boolean = false;
let nextUnitOfWork: Fiber | null = null;
let nextRoot: FiberRoot | null = null;
let nextRenderExpirationTime: ExpirationTime = NoWork;
let nextLatestAbsoluteTimeoutMs: number = -1;
let nextRenderDidError: boolean = false;
let nextEffect: Fiber | null = null;
let isCommitting: boolean = false;
let legacyErrorBoundariesThatAlreadyFailed: Set<mixed> | null = null;
 Used for performance tracking.
let interruptedBy: Fiber | null = null;
let stashedWorkInProgressProperties;
let replayUnitOfWork;
let isReplayingFailedUnitOfWork;
let originalReplayError;
let rethrowOriginalError;
if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
  stashedWorkInProgressProperties = null;
  isReplayingFailedUnitOfWork = false;
  originalReplayError = null;
  replayUnitOfWork = (
    failedUnitOfWork: Fiber,
    thrownValue: mixed,
    isYieldy: boolean,
  ) => {
    if (
      thrownValue !== null &&