Fix classes that shouldn't be extended/instantiated/mixedin

lib/ui/isolate_name_server.dart

@@ -20,9 +20,9 @@ part of dart.ui;
 /// recommended to establish a separate communication channel in that first
 /// message (e.g. by passing a dedicated [SendPort]).
 class IsolateNameServer {
-  // This class is only a namespace, and should not be instantiated or
-  // extended directly.
-  factory IsolateNameServer._() => throw UnsupportedError('Namespace');
+  // This class is not meant to be instantiated or extended; this constructor
+  // prevents instantiation and extension.
+  IsolateNameServer._();
 
   /// Looks up the [SendPort] associated with a given name.
   ///

lib/ui/natives.dart

@@ -7,6 +7,10 @@ part of dart.ui;
 
 /// Helper functions for Dart Plugin Registrants.
 class DartPluginRegistrant {
+  // This class is not meant to be instantiated or extended; this constructor
+  // prevents instantiation and extension.
+  DartPluginRegistrant._();
+
   static bool _wasInitialized = false;
 
   /// Makes sure the that the Dart Plugin Registrant has been called for this

lib/ui/painting.dart

@@ -3501,6 +3501,9 @@ class _ColorFilter extends NativeFieldWrapperClass1 {
 ///  * [SceneBuilder.pushImageFilter], which is the low-level API for using
 ///    this class as a child layer filter.
 abstract class ImageFilter {
+  // This class is not meant to be extended; this constructor prevents extension.
+  ImageFilter._(); // ignore: unused_element
+
   /// Creates an image filter that applies a Gaussian blur.
   factory ImageFilter.blur({ double sigmaX = 0.0, double sigmaY = 0.0, TileMode tileMode = TileMode.clamp }) {
     return _GaussianBlurImageFilter(sigmaX: sigmaX, sigmaY: sigmaY, tileMode: tileMode);

lib/ui/plugins.dart

@@ -40,9 +40,9 @@ class CallbackHandle {
 ///  * [IsolateNameServer], which provides utilities for dealing with
 ///    [Isolate]s.
 class PluginUtilities {
-  // This class is only a namespace, and should not be instantiated or
-  // extended directly.
-  factory PluginUtilities._() => throw UnsupportedError('Namespace');
+  // This class is not meant to be instantiated or extended; this constructor
+  // prevents instantiation and extension.
+  PluginUtilities._();
 
   static final Map<Function, CallbackHandle?> _forwardCache =
       <Function, CallbackHandle?>{};