gistfile1.js

  _waitForTransitionEnd: function(cardId) {
    var client = this.client;
    client.waitFor(function() {
      var condition = false;
      client.executeScript(
        function() {
          var Cards = window.wrappedJSObject.require('mail_common').Cards,
              card = Cards._cardStack[Cards.activeCardIndex],
              cardNode = card && card.domNode;

          return cardNode && cardNode.dataset.type === cardId &&
                 !Cards._eatingEventsUntilNextCard;
        },
        function(error, result) {
          if (result === true) {
            condition = true;
          }
        }
      );
      return condition;
    });
  },