Reg's Decorators as Mixins

function mixin (behaviour, sharedBehaviour = {}) {
  const instanceKeys = Reflect.ownKeys(behaviour);
  const sharedKeys = Reflect.ownKeys(sharedBehaviour);
  const typeTag = Symbol('isa');

  function _mixin (target) {
    for (let property of instanceKeys)
      Object.defineProperty(target, property, { value: behaviour[property] });
    Object.defineProperty(target, typeTag, { value: true });
    return target;
  }
  for (let property of sharedKeys)
    Object.defineProperty(_mixin, property, {
      value: sharedBehaviour[property],
      enumerable: sharedBehaviour.propertyIsEnumerable(property)
    });
  Object.defineProperty(_mixin, Symbol.hasInstance, {
    value: (i) => !!i[typeTag]
  });
  return _mixin;
}