some more fixes

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

                         new Expanded(
                             child: new GestureDetector(
                                 behavior: HitTestBehavior.opaque,
-                                onTap: () => { this.onChanged(this.value); },
+                                onTap: () =>
+                                {
+                                    this.onChanged(this.value);
+                                },
                                 child: new Text(this.value.title,
                                     style: theme.textTheme.subtitle1
                                 )
                                 width: 2.0f
                             )
                         ),
-                        child: new Text(
-                            namedColor.name
+                        child: new Container(
                         )
                     );
                 }).ToList()

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

 
   // https://en.wikipedia.org/wiki/Free_Four
   string dropdown1Value = "Free";
-  string dropdown2Value = "";
+  string dropdown2Value = null;
   string dropdown3Value = "Four";
 
   Widget buildDropdownButton() {

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/gallery/demos.cs

                     routeName: BottomNavigationDemo.routeName,
                     documentationUrl: "https://docs.flutter.io/flutter/material/BottomNavigationBar-class.html",
                     buildRoute: (BuildContext context) => new BottomNavigationDemo()
+                ),
+                new GalleryDemo(
+                    title: "Buttons",
+                    subtitle: "Flat, raised, dropdown, and more",
+                    icon: GalleryIcons.generic_buttons,
+                    category: GalleryDemoCategory._kMaterialComponents,
+                    routeName: ButtonsDemo.routeName,
+                    buildRoute: (BuildContext context) => new ButtonsDemo()
                 )
             };
 

com.unity.uiwidgets/Runtime/material/outline_button.cs

                 clipBehavior: clipBehavior,
                 focusNode: focusNode,
                 autofocus: autofocus,
+                icon: icon,
                 label: label
             );
         }

com.unity.uiwidgets/Runtime/material/raised_button.cs

             D.assert(icon != null);
             D.assert(label != null);
 
+            clipBehavior = clipBehavior ?? Clip.none;
+
             return new _RaisedButtonWithIcon(
                 key: key,
                 onPressed: onPressed,

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

         public virtual float? childMainAxisPosition(RenderObject child) {
             D.assert(() => { throw new UIWidgetsError(GetType() + " does not implement childPosition."); });
 
-            return null;
+            return 0.0f;
-            return null;
+            return 0.0f;
-            return null;
+            return 0.0f;
         }
 
         public override void applyPaintTransform(RenderObject child, Matrix4 transform) {