message: allow getting the button ref
diff: import React from "react";
  +import PropTypes from "prop-types";
  import classNames from "classnames";
  import "./style.css";
  export default class FlatButton extends React.Component {
  + static propTypes = {
  + buttonRef: PropTypes.func
  + };
  render() {
  - return (<button type="button" {...this.props} className={classNames("btn", this.props.className)} />); // eslint-disable-line react/prop-types
  + const props = {...this.props};
  + delete props.buttonRef;
  + return (<button type="button" ref={this.props.buttonRef} {...props} className={classNames("btn", this.props.className)} />); // eslint-disable-line react/prop-types
  }
  }
  