ensure order when using the then method in unit tests

[pflannery]

would be really nice if we could have the then calls ordered so we can put break points in between them, currently using setTimeout breaks the tests when there is one break point in them. Would be nice to replace setTimeout with Promises. like this:

// chainable then method
function then(callback) {
  var queue = this.queue;
  if (!queue)
    queue = this.queue = [];

  queue.push(Platform.performMicrotaskCheckpoint);
  queue.push(callback);

  return {
    then: then.bind(this),
    run: function (doneCallback) {
      var runnerPromise = Promise.resolve();
      queue.forEach(function (queuedPromise) {
        runnerPromise = runnerPromise.then(queuedPromise);
      });
      if (doneCallback)
        runnerPromise.then(doneCallback);
    }
  };
}

// example usage:
  new then(function() {
    console.log(1);
  }).then(function() {
    console.log(2);
  }).then(function() {
    console.log(3)
  }).then(function() {
    console.log(3)
  }).run(function() {
    console.log("complete");
  });