fix compiler errors

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsExample.cs

 using Unity.UIWidgets.animation;
 using Unity.UIWidgets.engine2;
 using Unity.UIWidgets.foundation;
-using Unity.UIWidgets.material;
+//using Unity.UIWidgets.material;
 using Unity.UIWidgets.painting;
 using Unity.UIWidgets.ui;
 using Unity.UIWidgets.widgets;

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/GalleryMain.cs

 using UIWidgetsGallery.gallery;
 using Unity.UIWidgets.engine2;
-using Unity.UIWidgets.material;
+//using Unity.UIWidgets.material;
 using Unity.UIWidgets.ui;
 using Unity.UIWidgets.widgets;
 using UnityEngine;

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/cupertino/cupertino_navigation_demo.cs

 //using RSG;
 using Unity.UIWidgets.cupertino;
 using Unity.UIWidgets.foundation;
-using Unity.UIWidgets.material;
+//using Unity.UIWidgets.material;
 using Unity.UIWidgets.painting;
 using Unity.UIWidgets.rendering;
 using Unity.UIWidgets.service;

com.unity.uiwidgets/Runtime/rendering/layer.cs

         
         bool  debugSubtreeNeedsAddToScene {
             get {
-                bool result;
+                bool result = false;
                 D.assert(()=>{
                     result = _needsAddToScene;
                     return true;