Using Polymers own behaviors in Polymer 2.x

I am experimenting with Polymer 2.x and what I don't understand is how to use Polymers own behaviors in Polymer 2.x, iron-resizable-behavior as an example.

The Polymer 2.0 upgrade guide tells us that for our own components we should work with class expression mixins. That's fine but what about Polymers own behaviors? Are they in the progress of being rewritten as mixins or will they remain unchanged? There is a Polymer.mixinBehaviors method that seems to allow me to use Polymer 1.x mixins. Is this the final solution or is this an intermediate step?

To phrase it another way: are the Polymer behaviors considered Polymer 2.x although we are told to use mixins for our own components?

Sources:

https://www.polymer-project.org/2.0/docs/upgrade#upgrading-to-class-based-elements

Applying Behaviors with JS Mixins in Polymer 2

I think Polymer has an API for that. I think it is dedupingMixin. Below is an example how to create a mixin for your own behavior and how to use it in your element class.

var MyBehaviorMixin = Polymer.dedupingMixin(function(superClass){
  return class MyBehavior extends superClass {
    constructor() {
      super();
    }

    methodInBehavior() {
      return "this method is defined in Behavior";
    }
  }
}
class MyElement extends MyBehaviorMixin(Polymer.Element){
  constructor(){
    super();
    console.log(this.methodInBehavior());
  }
}