es6mixin.js

var Bar1 = base => class extends base {
  componentWillMount(){
    super.componentWillMount();
    console.log('Bar1');
  }
};

var Bar2 = base => class extends base {
  componentWillMount(){
    super.componentWillMount();
    console.log('Bar2');
  }
};

class Foo extends mixins(Bar1, Bar2) {
  componentWillMount() {
    console.log('Foo before mixins');
    super.componentWillMount();
    console.log('Foo after mixins');
  }
}

mixins.js

function mixins(...mixinFactories) {
  var base = class {};
  // TODO: Add all possible method names that might call super()
  // to the base class so that they don't throw.
  for (var i = 0; i < mixinFactories.length; i++) {
     base = mixinFactories[i](base);
  }
  return base;
}