code: function Tweenable (opt_initialState, opt_config) {
        this._currentState = opt_initialState || {};
        this._configured = false;
        this._scheduleFunction = DEFAULT_SCHEDULE_FUNCTION;
  
        // To prevent unnecessary calls to setConfig do not set default configuration here.
        // Only set default configuration immediately before tweening if none has been set.
        if (typeof opt_config !== 'undefined') {
          this.setConfig(opt_config);
        }
      }
docs: Tweenable constructor.
  @param {Object=} opt_initialState The values that the initial tween should start at if a "from" object is not provided to Tweenable#tween.
  @param {Object=} opt_config See Tweenable.prototype.setConfig()
  @constructor