Hi, I'm having an issue with `onMouseEnter` being triggered on children of a component. *It only occurs with React 16*.

I've created a [CodePen that shows the issue](https://codepen.io/testprofile75/pen/VMXXdb).

Simplified structure is as below:
```
           <Card/> :
–––––––––––––––––––––––––––––
|                            |
|          <Cover/>          |
|                            |
------------------------------
|                            |
|    <div class="text" />    |
|                            |
–––––––––––––––––––––––––––––
```

`<Card />` is a component that needs to capture `onMouseMove` events (and trigger a `render`) and `<Cover/>` is a simple component that waits for an image (passed as a `prop`) to be loaded before displaying it (using `onload` on an `Image` created in the `constructor`).

If you open the console, you'll see that `onMouseEnter` is triggered when hovering `<Card />` (normal behavior), but also when hovering the `.text` div (which is not supposed to happen).

If you remove the whole `onload` logic from the `<Cover/>` element, then `onMouseEnter` works as intended.

It's possible that I'm coding this in a way that isn't appropriate, but it does look like a bug anyway :)


});
  it('should set relatedTarget properly in iframe', () => {
    var iframe = document.createElement('iframe');
    document.body.appendChild(iframe);
    var iframeDocument = iframe.contentDocument;
    iframeDocument.write(
      '<!DOCTYPE html><html><head></head><body><div></div></body></html>',
    );
    iframeDocument.close();
    var component = ReactDOM.render(
      <div />,
      iframeDocument.body.getElementsByTagName('div')[0],
    );
    var div = ReactDOM.findDOMNode(component);
    var extracted = EnterLeaveEventPlugin.extractEvents(
      'topMouseOver',
      ReactDOMComponentTree.getInstanceFromNode(div),
      {target: div},
      div,
    );
    expect(extracted.length).toBe(2);
    var leave = extracted[0];
    var enter = extracted[1];
    expect(leave.target).toBe(iframe.contentWindow);
    expect(leave.relatedTarget).toBe(div);
    expect(enter.target).toBe(div);
    expect(enter.relatedTarget).toBe(iframe.contentWindow);
  });
});