Listener('mousedown', this.onMouseDown_);
    this.target_ = undefined;
    this.horizontal = true;
    this.observer_ = new MutationObserver(
        this.didTargetMutate_.bind(this));
    this.targetSizesByModeKey_ = {};
    this.currentDraggingSize_ = undefined;
  },

  get modeKey_() {
    return this.target_.className === '' ? '.' : this.target_.className;
  },

  get target() {
    return this.target_;
  },

  set target(target) {
    this.observer_.disconnect();
    this.target_ = target;
    if (!th