I don't see this failure in our imported tests. Closing.

Reopening this, failure example at:

http://codepen.io/anon/pen/WGmObZ

This should throw an exception from define, but it does not.

'use strict';

let attrs = {};
attrs[Symbol.iterator] = function*() {
  throw 'bar';
}

class X extends HTMLElement {
  static get observedAttributes() {
    console.log('retrieving attrs');
    return attrs;
  }
  attributeChangedCallback(name) {
    console.log(`${name} changed`)
  }
}

customElements.define('x-x', X);

This should be fixed by bashi's patch 19f37fd06b24d3af5e84f019e561d18c125a29e9