testStartDrag_LeftMouseDownOnly() {
    const dragger = new Dragger(target);

    const e = new StrictMock(BrowserEvent);
    /**
     * @suppress {strictMissingProperties} suppression added to enable type
     * checking
     */
    e.type = EventType.MOUSEDOWN;
    /**
     * @suppress {strictMissingProperties} suppression added to enable type
     * checking
     */
    e.clientX = 1;
    /**
     * @suppress {strictMissingProperties} suppression added to enable type
     * checking
     */
    e.clientY = 2;
    e.isMouseActionButton().$returns(false);
    e.$replay();

    events.listen(dragger, Dragger.EventType.START, (e) => {
      fail('No drag START event should have been dispatched');
    });

    dragger.startDrag(e);

    assertFalse('Start drag must have been cancelled.', dragger.isDragging());
    assertFalse(
        'Dragger must not have registered mousemove handlers.',
        events.hasListener(dragger.document_, EventType.MOUSEMOVE, true));
    assertFalse(
        'Dragger must not have registered mouseup handlers.',
        events.hasListener(dragger.document_, EventType.MOUSEUP, true));
    e.$verify();
  }
Tests that start drag happens on left mousedown.
     @suppress {missingProperties,checkTypes,visibility} suppression added to
     enable type checking