This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
static contextTypes = {
        test: PropTypes.string.isRequired,
      };
      render() {
        return <div>{this.context.test}</div>;
      }
    }
    function Parent() {
      return <Child />;
    }
    class GrandParent extends React.Component {
      static childContextTypes = {
        test: PropTypes.string.isRequired,
      };
      getChildContext() {