bug

Warning displayed in console about internal error of React performance measurement code.

``` js
import React from 'react';
import { render } from 'react-dom';
import Perf from 'react-addons-perf';
import { browserHistory, Router } from 'react-router';

import routes from './routes';
import persist from 'altFlux/persist';

persist('docs');

const target = document.getElementById('app');

Perf.start();

render(
  <Router
    routes={routes}
    history={browserHistory}
  />,
  target,
  () => {
    Perf.stop();
    Perf.printWasted();
  }
);
```

![screen shot 2016-05-23 at 9 21 08 am](https://cloud.githubusercontent.com/assets/91464/15472297/5e440d48-20c8-11e6-99d7-363f6d1bdcce.png)

15.1.0
Google Chrome

'`ReactPerf.printOperations(...)` instead.'
    );
    ReactPerf.printDOM(measurements);
    expect(console.error.calls.length).toBe(1);
  });
  it('returns isRunning state', () => {
    expect(ReactPerf.isRunning()).toBe(false);
    ReactPerf.start();
    expect(ReactPerf.isRunning()).toBe(true);
    ReactPerf.stop();
    expect(ReactPerf.isRunning()).toBe(false);
  });
  it('start has no effect when already running', () => {
    expect(ReactPerf.isRunning()).toBe(false);
    ReactPerf.start();
    expect(ReactPerf.isRunning()).toBe(true);
    ReactPerf.start();
    expect(ReactPerf.isRunning()).toBe(true);
    ReactPerf.stop();
    expect(ReactPerf.isRunning()).toBe(false);
  });
  it('stop has no effect when already stopped', () => {
    expect(ReactPerf.isRunning()).toBe(false);
    ReactPerf.stop();
    expect(ReactPerf.isRunning()).toBe(false);
    ReactPerf.stop();
    expect(ReactPerf.isRunning()).toBe(false);
  });
});