I've tested the new Perf tools on one of the screens in one app I'm working on and it works great :)

One thing I wanted also on the old one Perf tools was to be able to check whether the Perf tools are started or stopped, so basically exposing `ReactDebugTool`'s `isProfiling` variable. My use for that is that I have a keyboard shortcut (in development) bound to start/stop the Perf tools and currently I'm forced to maintain such a variable myself to know whether to start or stop (which gets out of sync if I start/stop the perf tools without the keyboard shortcut).

If you don't think it's worthwhile to expose `isProfiling` feel free to close this issue.

var measurements = measure(fn);
    var summary = ReactPerf.getWasted(measurements);
    expect(summary).toEqual([]);
  }
  it('should not count initial render as waste', function() {
    expectNoWaste(() => {
      ReactTestUtils.renderIntoDocument(<App />);
    });
  });
  it('should not count unmount as waste', function() {
    var container = document.createElement('div');
    ReactDOM.render(<Div>hello</Div>, container);
    expectNoWaste(() => {
      ReactDOM.unmountComponentAtNode(container);
    });
  });
  it('should not count content update as waste', function() {
    var container = document.createElement('div');
    ReactDOM.render(<Div>hello</Div>, container);
    expectNoWaste(() => {
      ReactDOM.render(<Div>hello world</Div>, container);
    });
  });
  it('should not count child addition as waste', function() {
    var container = document.createElement('div');
    ReactDOM.render(<Div><span /></Div>, container);
    expectNoWaste(() => {
      ReactDOM.render(<Div><span /><span /></Div>, container);