作ってみたけど、あまり格好良い __super__ じゃなかったorz

extend.js

var [mixin, extend] = (function() {
  var wm = WeakMap();
  function mixin(aTarget, ...aSources) {
    for (let source of aSources) {
      for (let key of Object.getOwnPropertyNames(source)) {
        let desc = Object.getOwnPropertyDescriptor(source, key);
        Object.defineProperty(aTarget, key, desc);
      }
    }
    return aTarget;
  }
  function extend (aClass, ...aSources) {
    var supers = [];
    var proto = aSources.shift();
    if (typeof proto === "function") {
      if (proto.name)
        supers[proto.name] = proto.prototype;

      proto = proto.prototype;
    }
    supers.unshift(proto);

    for (let source of aSources) {
      let target = Object.create(proto);
      if (typeof source ===  "function") {
        if (source.name)
          supers[source.name] = source.prototype;

        source = source.prototype;
      }
      supers.unshift(source);
      proto = mixin(target, source);
    }
    wm.set(aClass, Object.freeze(supers));
    aClass.prototype = mixin(proto, aClass.prototype);
    Object.defineProperty(aClass.prototype, "__super__", { get: function(){ return wm.get(aClass); } });
    return aClass.prototype;
  }
  return [mixin, extend];
}());

// example
function Base(...args){ this.init.apply(this, args); }
mixin(Base.prototype, {
  init: function (name) {
    this.name = name;
  },
  getName: function(){
    return this.name;
  },
});

function Foo(...args){
  Base.apply(this, args);
}
extend(Foo, Base, {
  setName: function(val) {
    this.name = val;
  },
  getName: function() {
    var name = this.__super__.Base.getName.call(this);
    return "Foo: " + name;
  },
});

// test
var f = new Foo("hogehoge");
console.log(f.getName());