on(done) {
        var f = fixture('basic');
        var target = f.querySelector('#target');
        var tooltip = f.querySelector('paper-tooltip');

        var actualTooltip = Polymer.dom(tooltip.root).querySelector('#tooltip');
        assert.isTrue(isHidden(actualTooltip));
        // Simulate but don't actually run the entry animation.
        tooltip.toggleClass('hidden', false, actualTooltip);
        tooltip._showing = true;
        assert.isFalse(isHidden(actualTooltip));

        tooltip.addEvent