Using Polymers own behaviors in Polymer 2.x

class MyElement extends Polymer.mixinBehaviors([Polymer.IronFormElementBehavior], Polymer.Element) { ... }

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());
  }
}

So I found a way how to use the iron-resizable-behavior in Polymer2
Instead of using the Mixin I am making use of the syntax of codeMonkey and extended his answer to show how to use it
As far as I know, this solution is a hybrid for Polymer 1 & 2

class customElement extends Polymer.mixinBehaviors([Polymer.IronResizableBehavior], Polymer.Element) {
    static get is() { return 'custom-element'; }

    ready() {
        super.ready();

        this.addEventListener('iron-resize', () => { this.onWidthChange() });

    }

    onWidthChange() {
        var w = this.offsetWidth;
        if (w > 0)
            console.log("Width of page is now ", w);
    }

}