fix some runtime errors (for CountDemo sample)

Samples/UIWidgetsSamples_2019_4/Assets/CountDemo.cs

 using UnityEngine.UI;
 using Text = Unity.UIWidgets.widgets.Text;
 using ui_ = Unity.UIWidgets.widgets.ui_;
+using TextStyle = Unity.UIWidgets.painting.TextStyle;
 
 namespace UIWidgetsSample
 {
         public override Widget build(BuildContext context)
         {
             return new Container(
-                color: Color.white,
+                color: Color.fromARGB(255, 255, 0, 0),
-                        new Text($"count: {count}"),
+                        new Text($"count: {count}", style: new TextStyle(color: Color.fromARGB(255, 0 ,0 ,255))),
                         new CupertinoButton(
                             onPressed: () =>
                             {
                                 });
                             },
                             child: new Container(
-                                color: Color.black,
+                                color: Color.fromARGB(255,0 , 255, 0),
                                 width: 100,
                                 height: 40
                             )

Samples/UIWidgetsSamples_2019_4/Assets/CountDemo.unity

   m_AnchorMin: {x: 0.5, y: 0.5}
   m_AnchorMax: {x: 0.5, y: 0.5}
   m_AnchoredPosition: {x: 0, y: 0}
-  m_SizeDelta: {x: 100, y: 100}
+  m_SizeDelta: {x: 500, y: 500}
   m_Pivot: {x: 0.5, y: 0.5}
 --- !u!222 &1777874942
 CanvasRenderer:
   m_GameObject: {fileID: 1777874940}
   m_Enabled: 1
   m_EditorHideFlags: 0
-  m_Script: {fileID: 11500000, guid: 169ec79ce040653478e1a7725f34bbe1, type: 3}
+  m_Script: {fileID: 11500000, guid: 0817e3443c80cb943a03dcf7b120bd2e, type: 3}
-  m_RaycastTarget: 0
+  m_RaycastTarget: 1
   m_Maskable: 1
   m_OnCullStateChanged:
     m_PersistentCalls:

com.unity.uiwidgets/Runtime/painting/text_style.cs

             return new TextStyle(
                 inherit: inherit ?? this.inherit,
                 color: this.foreground == null && foreground == null ? color ?? this.color : null,
-                backgroundColor: this.background == null && background == null ? color ?? this.backgroundColor : null,
+                backgroundColor: this.background == null && background == null ? backgroundColor ?? this.backgroundColor : null,
                 fontFamily: fontFamily ?? this.fontFamily,
                 fontFamilyFallback: fontFamilyFallback ?? this.fontFamilyFallback,
                 fontSize: fontSize ?? this.fontSize,

com.unity.uiwidgets/Runtime/widgets/app.cs

             routes = routes ?? new Dictionary<string, WidgetBuilder>();
             supportedLocales = supportedLocales ?? new List<Locale> {new Locale("en", "US")};
             window = Window.instance;
-            D.assert(navigatorObservers != null);
             D.assert(routes != null);
             this.home = home;
             this.navigatorKey = navigatorKey;

com.unity.uiwidgets/Runtime/widgets/basic.cs

         /// The [color] parameter must not be null.
         public ColoredBox(Color color = null, Widget child = null, Key key = null) : base(key: key, child: child) {
             D.assert(color != null);
+            this.color = color;
         }
         /// The color to paint the background area with.
         public readonly Color color;

com.unity.uiwidgets/Runtime/widgets/navigator.cs

                 widget.pages.isEmpty() || widget.onPopPage != null, () =>
                     "The Navigator.onPopPage must be provided to use the Navigator.pages API"
             );
-            if (((Navigator)oldWidget).observers != widget.observers) {
+            if (!((Navigator)oldWidget).observers.equalsList(widget.observers)) {
                 foreach (NavigatorObserver observer in ((Navigator)oldWidget).observers)
                     observer._navigator = null;
                 foreach (NavigatorObserver observer in widget.observers) {
             }
 
-            if (((Navigator)oldWidget).pages != widget.pages) {
+            if (!((Navigator)oldWidget).pages.equalsList(widget.pages)) {
                 D.assert(
                     widget.pages.isNotEmpty(), () =>
                         "To use the Navigator.pages, there must be at least one page in the list."

com.unity.uiwidgets/Runtime/widgets/pages.cs

             bool fullscreenDialog = false
         ) : base(settings,fullscreenDialog) {
             D.assert(pageBuilder != null);
-            D.assert(transitionsBuilder != null);
-            
+
             this.opaque = opaque;
             this.pageBuilder = pageBuilder;
             this.transitionsBuilder = transitionsBuilder ?? PagesUtils._defaultTransitionsBuilder;