版本（versions）：
react@16.0.0
react-dom@16.0.0

描述（description）：
给父级别绑定 onMouseEnter 事件。在首次渲染后，鼠标移入新渲染进去的子元素会触发父元素的 onMouseEnter 事件。

示例（demo）：

```js
import React, {Component} from 'react';

class Demo extends Component {
    constructor(prop) {
        super(prop);
        this.enter = this.enter.bind(this)
        this.state = {
            flag: false
        }
    }
    enter(event) {
        console.log(event.nativeEvent);
        console.log("enter")
    }
    click() {
        this.setState({flag:!this.state.flag})
    }
    render() {
        let old = {
            height: "100px",
            width: "100px",
            border: "1px solid #6dbbff"
        };
        let aa = {
            height: "100px",
            width: "100px",
            border: "1px solid red"
        };
        let content = {
            border: "1px solid black",
            display: "flex",
            padding:"10px"
        }
        return(
            <div>
                <div style={content} onMouseEnter={this.enter}>
                    {this.state.a}
                    <div style={old}>old</div>
                    <div style={aa}>old</div>
                    {this.state.flag && <div style={aa}>new</div>}
                </div>
                <button onClick={this.click.bind(this)}>点我（click me）</button>
            </div>
        )
    }
}
```
});
  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);
  });
});