EmitChangesDirective should use __classProperties

[hotforfeature]

EmitChangesDirective uses some weirdness to get all properties and behavior properties for an element. Some digging revealed that Polymer actually creates a flattened __classProperties object that contains all the properties going up the chain.

I think the current method is working for hybrid elements, but will fail in this scenario with mixins:

MyMixin = Polymer.dedupingMixin(base => {
  return class MyMixin extends base {
    static get properties() {
      return {
        totallyLegal: {
          type: Boolean,
          value: true,
          reflectToAttribute: true
        };
      };
    }
  }
});

class MyElement extends MyMixin(Polymer.Element) {
  static get properties() {
    return {
      ownProp: {
        type: Boolean,
        value: true,
        reflectToAttribute: true
      }
    };
  }
  
  connectedCallback() {
    console.log(this.ownProp); // true
    console.log(this.totallyLegal); // true
  }
}

I don't think Origami will pick up on the totallyLegal, which is totally wrong.