gistfile1.js

let { promise } = require('sdk/window/helpers');
  console.log("foo1");
  promise(open('chrome://foo/content/'), 'load').then(function(window) {
    console.log("foo2");
    assert.ok(~windows().indexOf(window), 'chrome URI works');
    console.log("foo3");
    return window; // pass window to next function
  }).then(close).
    then(promise.bind(null, 'resource://foo', 'load')).then(function(window) {
      assert.ok(~windows().indexOf(window), 'resource URI works');
      console.log("foo4");
      return window;
    }).then(close).then(done);