some fixes on image file path

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/material/backdrop_demo.cs

                 title: "Accessories",
                 assets: new List<string>
                 {
-                    "StreamingAssets/gallery/products/belt.png",
-                    "StreamingAssets/gallery/products/earrings.png",
-                    "StreamingAssets/gallery/products/backpack.png",
-                    "StreamingAssets/gallery/products/hat.png",
-                    "StreamingAssets/gallery/products/scarf.png",
-                    "StreamingAssets/gallery/products/sunnies.png"
+                    "gallery/products/belt.png",
+                    "gallery/products/earrings.png",
+                    "gallery/products/backpack.png",
+                    "gallery/products/hat.png",
+                    "gallery/products/scarf.png",
+                    "gallery/products/sunnies.png"
                 }
             ),
             new Category(
-                    "StreamingAssets/gallery/products/backpack.png",
-                    "StreamingAssets/gallery/products/cup.png",
-                    "StreamingAssets/gallery/products/napkins.png",
-                    "StreamingAssets/gallery/products/top.png"
+                    "gallery/products/backpack.png",
+                    "gallery/products/cup.png",
+                    "gallery/products/napkins.png",
+                    "gallery/products/top.png"
                 }
             ),
             new Category(
-                    "StreamingAssets/gallery/products/jacket.png",
-                    "StreamingAssets/gallery/products/jumper.png",
-                    "StreamingAssets/gallery/products/scarf.png",
-                    "StreamingAssets/gallery/products/sweater.png",
-                    "StreamingAssets/gallery/products/sweats.png"
+                    "gallery/products/jacket.png",
+                    "gallery/products/jumper.png",
+                    "gallery/products/scarf.png",
+                    "gallery/products/sweater.png",
+                    "gallery/products/sweats.png"
                 }
             ),
             new Category(
-                    "StreamingAssets/gallery/products/cup.png",
-                    "StreamingAssets/gallery/products/napkins.png",
-                    "StreamingAssets/gallery/products/planters.png",
-                    "StreamingAssets/gallery/products/table.png",
-                    "StreamingAssets/gallery/products/teaset.png"
+                    "gallery/products/cup.png",
+                    "gallery/products/napkins.png",
+                    "gallery/products/planters.png",
+                    "gallery/products/table.png",
+                    "gallery/products/teaset.png"
                 }
             ),
             new Category(
-                    "StreamingAssets/gallery/products/jumper.png",
-                    "StreamingAssets/gallery/products/shirt.png",
-                    "StreamingAssets/gallery/products/sweater.png",
-                    "StreamingAssets/gallery/products/top.png"
+                    "gallery/products/jumper.png",
+                    "gallery/products/shirt.png",
+                    "gallery/products/sweater.png",
+                    "gallery/products/top.png"
                 }
             ),
             new Category(
-                    "StreamingAssets/gallery/products/backpack.png",
-                    "StreamingAssets/gallery/products/belt.png",
-                    "StreamingAssets/gallery/products/cup.png",
-                    "StreamingAssets/gallery/products/dress.png",
-                    "StreamingAssets/gallery/products/earrings.png",
-                    "StreamingAssets/gallery/products/flatwear.png",
-                    "StreamingAssets/gallery/products/hat.png",
-                    "StreamingAssets/gallery/products/jacket.png",
-                    "StreamingAssets/gallery/products/jumper.png",
-                    "StreamingAssets/gallery/products/napkins.png",
-                    "StreamingAssets/gallery/products/planters.png",
-                    "StreamingAssets/gallery/products/scarf.png",
-                    "StreamingAssets/gallery/products/shirt.png",
-                    "StreamingAssets/gallery/products/sunnies.png",
-                    "StreamingAssets/gallery/products/sweater.png",
-                    "StreamingAssets/gallery/products/sweats.png",
-                    "StreamingAssets/gallery/products/table.png",
-                    "StreamingAssets/gallery/products/teaset.png",
-                    "StreamingAssets/gallery/products/top.png"
+                    "gallery/products/backpack.png",
+                    "gallery/products/belt.png",
+                    "gallery/products/cup.png",
+                    "gallery/products/dress.png",
+                    "gallery/products/earrings.png",
+                    "gallery/products/flatwear.png",
+                    "gallery/products/hat.png",
+                    "gallery/products/jacket.png",
+                    "gallery/products/jumper.png",
+                    "gallery/products/napkins.png",
+                    "gallery/products/planters.png",
+                    "gallery/products/scarf.png",
+                    "gallery/products/shirt.png",
+                    "gallery/products/sunnies.png",
+                    "gallery/products/sweater.png",
+                    "gallery/products/sweats.png",
+                    "gallery/products/table.png",
+                    "gallery/products/teaset.png",
+                    "gallery/products/top.png"
                 }
             )
         };

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/material/bottom_app_bar_demo.cs

 
         public override Path getOuterPath(Rect host, Rect guest)
         {
-            if (!host.overlaps(guest))
+            //there is a bug in flutter when guest == null, we fix it here
+            if (guest == null || !host.overlaps(guest))
             {
                 var path = new Path();
                 path.addRect(host);

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/material/cards_demo.cs

 {
     public static class CardsDemoUtils
     {
-        public static readonly string _kGalleryAssetsPackage = "StreamingAssets/gallery/";
+        public static readonly string _kGalleryAssetsPackage = "gallery/";
     }
 
     internal enum CardDemoType

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/material/chip_demo.cs

 
         public static readonly Dictionary<string, string> _avatars = new Dictionary<string, string>()
         {
-            {"hammer", "StreamingAssets/gallery/people/square/ali.png"},
-            {"chisel", "StreamingAssets/gallery/people/square/sandra.png"},
-            {"fryer", "StreamingAssets/gallery/people/square/trevor.png"},
-            {"fabricator", "StreamingAssets/gallery/people/square/stella.png"},
-            {"customer", "StreamingAssets/gallery/people/square/peter.png"},
+            {"hammer", "gallery/people/square/ali.png"},
+            {"chisel", "gallery/people/square/sandra.png"},
+            {"fryer", "gallery/people/square/trevor.png"},
+            {"fabricator", "gallery/people/square/stella.png"},
+            {"customer", "gallery/people/square/peter.png"},
         };
 
         public static readonly Dictionary<string, HashSet<string>> _toolActions =

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

 
         public static Rect inverseTransformRect(Matrix4 transform, Rect rect) {
             D.assert(rect != null);
+            
+            //FIXME: there is a bug here
+            //In flutter this assertion has been commented, but we cannot do it since the call of Matrix4.tryInvert will fail
+            //we need to find a better way to fix this issue here
             D.assert(transform.determinant() != 0.0);
             if (isIdentity(transform))
                 return rect;