How to extend React component class created via React.createClass

I have an existing React component created by React.createClass method. I want to reuse that React component by using class extend feature in ES6 (for method overriding purpose) but the result is unexpected. Example:

//an existing React component created via React.createClass
var ab = React.createClass({
    say(){
        console.log('ab class')
    },
    render(){
        return null;
    }
});

class c1 extends ab {
    say(){
        console.log('c1 class')
    };
}

o = new c1();
console.log(o.say()); //The expected console output is "c1 class" 
                      //but its showing "ab class"