Adds createRef() as per RFC

[trueadm]

This PR adds the React.createRef() API – which can be thought of as a replacement for those who use string refs or for those who don't need the powerful features from callback refs. To find out more about this new API, see its respective RFC: reactjs/rfcs#17.

Example:

class MyComponent extends React.Component {
  divRef = React.createRef();

  render() {
    return <div><input type="text" ref={this.divRef} /></div>;
  }

  componentDidMount() {
    this.divRef.current.focus();
  }
}

[bvaughn]

LGTM

[bvaughn]

Probably don't need to include this file?

[TrySound]

Wow, great! Are you gonna release this feature in 16.3?

[trueadm]

@TrySound Pending final comments and considerations on the RFC thread. :)

[JerseyW]

Uncaught TypeError: React.createRef is not a function ----- what is the problem??? my React is v16.2.0 ;
there is my code :

class  Demo extends  React.Component{

    myRefs = React.createRef();
    render() {

        return (
            <div>
                <label>

                    <input ref={ this.myRefs } type="text" placeholder="click button show data"/>
                </label>
                <button ref="btn" onClick={this.btnClick}>click</button>
                <label>
                    <input ref={this.myRefs } onBlur={this.show} type="text" placeholder="The data is out of focus"/>
                </label>
            </div>

        )
    }

    btnClick = ()=>{
       console.log(this.myRefs.current);
    }
    show = ()=>{

        console.log(this.myRefs);
    }
}



ReactDOM.render(<Demo />,document.getElementById("test1"));