Be more consistent about how we mark classes that can't be extended.

Fixes #26095

packages/flutter/lib/src/rendering/layout_helper.dart

@@ -18,6 +18,8 @@ typedef ChildLayouter = Size Function(RenderBox child, BoxConstraints constraint
 ///
 /// All of the functions adhere to the [ChildLayouter] signature.
 class ChildLayoutHelper {
+  // This class is not meant to be instantiated or extended; this constructor
+  // prevents instantiation and extension.
   const ChildLayoutHelper._();
 
   /// Returns the [Size] that the [RenderBox] would have if it were to

packages/flutter/lib/src/rendering/viewport.dart

@@ -32,7 +32,7 @@ enum CacheExtentStyle {
 abstract class RenderAbstractViewport extends RenderObject {
   // This class is intended to be used as an interface, and should not be
   // extended directly; this constructor prevents instantiation and extension.
-  factory RenderAbstractViewport._() => throw Error();
+  RenderAbstractViewport._();
 
   /// Returns the [RenderAbstractViewport] that most tightly encloses the given
   /// render object.

packages/flutter/lib/src/services/autofill.dart

@@ -12,6 +12,8 @@ import 'text_input.dart';
 /// documentation for their availability on each platform, and the platform
 /// values each autofill hint corresponds to.
 class AutofillHints {
+  // This class is not meant to be instantiated or extended; this constructor
+  // prevents instantiation and extension.
   AutofillHints._();
 
   /// The input field expects an address locality (city/town).

packages/flutter/lib/src/services/mouse_cursor.dart

@@ -417,7 +417,7 @@ class SystemMouseCursor extends MouseCursor {
 class SystemMouseCursors {
   // This class only contains static members, and should not be instantiated or
   // extended.
-  factory SystemMouseCursors._() => throw Error();
+  SystemMouseCursors._();
 
   // The mapping in this class must be kept in sync with the following files in
   // the engine: