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 :)


/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @emails react-core
 */
'use strict';
var EnterLeaveEventPlugin;
var React;
var ReactDOM;
var ReactDOMComponentTree;
describe('EnterLeaveEventPlugin', () => {
  beforeEach(() => {
    jest.resetModules();
    React = require('react');
    ReactDOM = require('react-dom');
    
    ReactDOMComponentTree = require('ReactDOMComponentTree');
    EnterLeaveEventPlugin = require('EnterLeaveEventPlugin');