Fix warnings and build errors

Fixes errors from #1285 and #1281.

packages/flutter/lib/src/fn3/basic.dart

@@ -119,7 +119,7 @@ class ClipRect extends OneChildRenderObjectWidget {
 
   RenderClipRect createRenderObject() => new RenderClipRect();
 
-  void updateRenderObject(ClipRect renderObject, ClipRect oldWidget) {
+  void updateRenderObject(RenderClipRect renderObject, ClipRect oldWidget) {
     // Nothing to update
   }
 }
@@ -145,7 +145,7 @@ class ClipOval extends OneChildRenderObjectWidget {
 
   RenderClipOval createRenderObject() => new RenderClipOval();
 
-  void updateRenderObject(ClipOval renderObject, ClipOval oldWidget) {
+  void updateRenderObject(RenderClipOval renderObject, ClipOval oldWidget) {
     // Nothing to update
   }
 }
@@ -464,7 +464,7 @@ class Stack extends MultiChildRenderObjectWidget {
 
   RenderStack createRenderObject() => new RenderStack();
 
-  void updateRenderObject(ClipRect renderObject, Stack oldWidget) {
+  void updateRenderObject(RenderStack renderObject, Stack oldWidget) {
     // Nothing to update
   }
 
@@ -747,7 +747,7 @@ class IgnorePointer extends OneChildRenderObjectWidget {
 
   final bool ignoring;
 
-  RenderIgnorePointer createElement() => new RenderIgnorePointer(ignoring: ignoring);
+  RenderIgnorePointer createRenderObject() => new RenderIgnorePointer(ignoring: ignoring);
 
   void updateRenderObject(RenderIgnorePointer renderObject, IgnorePointer oldWidget) {
     renderObject.ignoring = ignoring;

packages/flutter/lib/src/fn3/binding.dart

@@ -11,7 +11,7 @@ class WidgetSkyBinding extends SkyBinding {
 
   WidgetSkyBinding({ RenderView renderViewOverride: null })
     : super(renderViewOverride: renderViewOverride) {
-    BuildableElement.scheduleBuildFor = this.scheduleBuildFor;
+    BuildableElement.scheduleBuildFor = scheduleBuildFor;
   }
 
   /// Ensures that there is a SkyBinding object instantiated.

packages/flutter/lib/src/fn3/framework.dart

@@ -519,6 +519,7 @@ abstract class RenderObjectElement<T extends RenderObjectWidget> extends Element
   void unmount() {
     super.unmount();
     _widget.didUnmountRenderObject(renderObject);
+    _registry.remove(renderObject);
   }
 
   void detachRenderObject() {
@@ -528,11 +529,6 @@ abstract class RenderObjectElement<T extends RenderObjectWidget> extends Element
     }
   }
 
-  void unmount() {
-    super.unmount();
-    _registry.remove(renderObject);
-  }
-
   void insertChildRenderObject(RenderObject child, dynamic slot);
 
   void removeChildRenderObject(RenderObject child);