Remove private constructors for wrapped classes

Author: bkonyi

lib/ui/compositing.dart

@@ -17,7 +17,7 @@ class Scene extends NativeFieldWrapperClass2 {
   ///
   /// To create a Scene object, use a [SceneBuilder].
   @pragma('vm:entry-point')
-  Scene._();
+  Scene();
 
   /// Creates a raster image representation of the current state of the scene.
   /// This is a slow operation that is performed on a background thread.

lib/ui/painting.dart

@@ -1865,7 +1865,7 @@ class EngineLayer extends NativeFieldWrapperClass2 {
   /// This class is created by the engine, and should not be instantiated
   /// or extended directly.
   @pragma('vm:entry-point')
-  EngineLayer._();
+  EngineLayer();
 }
 
 /// A complex, one-dimensional subset of a plane.
@@ -4099,7 +4099,7 @@ class Picture extends NativeFieldWrapperClass2 {
   ///
   /// To create a [Picture], use a [PictureRecorder].
   @pragma('vm:entry-point')
-  Picture._();
+  Picture();
 
   /// Creates an image from this picture.
   ///

lib/ui/semantics.dart

@@ -820,7 +820,7 @@ class SemanticsUpdate extends NativeFieldWrapperClass2 {
   ///
   /// To create a SemanticsUpdate object, use a [SemanticsUpdateBuilder].
   @pragma('vm:entry-point')
-  SemanticsUpdate._();
+  SemanticsUpdate();
 
   /// Releases the resources used by this semantics update.
   ///