fixed ts-build failure
fixed ts-build failure

likangning93/cesium,CesiumGS/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium,likangning93/cesium