Mark unused but desired private constructors as such (#46200)

examples/flutter_gallery/lib/gallery/icons.dart

@@ -5,6 +5,9 @@
 import 'package:flutter/material.dart';
 
 class GalleryIcons {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   GalleryIcons._();
 
   static const IconData tooltip = IconData(0xe900, fontFamily: 'GalleryIcons');

packages/flutter/lib/src/animation/curves.dart

@@ -517,6 +517,9 @@ class ElasticInOutCurve extends Curve {
 ///  * [Curve], the interface implemented by the constants available from the
 ///    [Curves] class.
 class Curves {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   Curves._();
 
   /// A linear animation curve.

packages/flutter/lib/src/cupertino/colors.dart

@@ -18,6 +18,9 @@ import 'theme.dart';
 /// A palette of [Color] constants that describe colors commonly used when
 /// matching the iOS platform aesthetics.
 class CupertinoColors {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   CupertinoColors._();
 
   /// iOS 13's default blue color. Used to indicate active elements such as

packages/flutter/lib/src/cupertino/icons.dart

@@ -26,6 +26,9 @@ import 'package:flutter/widgets.dart';
 ///  * [Icon], used to show these icons.
 ///  * <https://github.com/flutter/cupertino_icons/blob/master/map.png>, a map of the icons in this icons font.
 class CupertinoIcons {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   CupertinoIcons._();
 
   /// The icon font used for Cupertino icons.

packages/flutter/lib/src/foundation/assertions.dart

@@ -112,6 +112,7 @@ class ErrorDescription extends _ErrorDiagnostic {
 
   /// Calls to the default constructor may be rewritten to use this constructor
   /// in debug mode using a kernel transformer.
+  // ignore: unused_element
   ErrorDescription._fromParts(List<Object> messageParts) : super._fromParts(messageParts, level: DiagnosticLevel.info);
 }
 
@@ -146,6 +147,7 @@ class ErrorSummary extends _ErrorDiagnostic {
 
   /// Calls to the default constructor may be rewritten to use this constructor
   /// in debug mode using a kernel transformer.
+  // ignore: unused_element
   ErrorSummary._fromParts(List<Object> messageParts) : super._fromParts(messageParts, level: DiagnosticLevel.summary);
 }
 
@@ -176,6 +178,7 @@ class ErrorHint extends _ErrorDiagnostic {
 
   /// Calls to the default constructor may be rewritten to use this constructor
   /// in debug mode using a kernel transformer.
+  // ignore: unused_element
   ErrorHint._fromParts(List<Object> messageParts) : super._fromParts(messageParts, level:DiagnosticLevel.hint);
 }
 

packages/flutter/lib/src/foundation/licenses.dart

@@ -288,6 +288,9 @@ class LicenseEntryWithLineBreaks extends LicenseEntry {
 ///  * [AboutListTile], which is a widget that can be added to a [Drawer]. When
 ///    tapped it calls [showAboutDialog].
 class LicenseRegistry {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   LicenseRegistry._();
 
   static List<LicenseEntryCollector> _collectors;

packages/flutter/lib/src/foundation/unicode.dart

@@ -11,6 +11,9 @@
 ///  * <http://unicode.org/reports/tr9/>, which describes the Unicode
 ///    bidirectional text algorithm.
 class Unicode {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   Unicode._();
   /// U+202A LEFT-TO-RIGHT EMBEDDING
   ///

packages/flutter/lib/src/gestures/converter.dart

@@ -32,6 +32,9 @@ int _synthesiseDownButtons(int buttons, PointerDeviceKind kind) {
 /// [dart:ui.Window.onPointerDataPacket], and converts them to [PointerEvent]
 /// objects.
 class PointerEventConverter {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   PointerEventConverter._();
 
   /// Expand the given packet of pointer data into a sequence of framework

packages/flutter/lib/src/gestures/hit_test.dart

@@ -11,8 +11,9 @@ import 'events.dart';
 
 /// An object that can hit-test pointers.
 abstract class HitTestable {
-  // This class is intended to be used as an interface with the implements
-  // keyword, and should not be extended directly.
+  // This class is intended to be used as an interface, and should not be
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory HitTestable._() => null;
 
   /// Check whether the given position hits this object.
@@ -24,8 +25,9 @@ abstract class HitTestable {
 
 /// An object that can dispatch events.
 abstract class HitTestDispatcher {
-  // This class is intended to be used as an interface with the implements
-  // keyword, and should not be extended directly.
+  // This class is intended to be used as an interface, and should not be
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory HitTestDispatcher._() => null;
 
   /// Override this method to dispatch events.
@@ -34,8 +36,9 @@ abstract class HitTestDispatcher {
 
 /// An object that can handle events.
 abstract class HitTestTarget {
-  // This class is intended to be used as an interface with the implements
-  // keyword, and should not be extended directly.
+  // This class is intended to be used as an interface, and should not be
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory HitTestTarget._() => null;
 
   /// Override this method to receive events.

packages/flutter/lib/src/material/chip.dart

@@ -63,7 +63,8 @@ const Icon _kDefaultDeleteIcon = Icon(Icons.cancel, size: _kDeleteIconSize);
 ///  * <https://material.io/design/components/chips.html>
 abstract class ChipAttributes {
   // This class is intended to be used as an interface, and should not be
-  // extended directly.
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory ChipAttributes._() => null;
 
   /// The primary content of the chip.
@@ -174,7 +175,8 @@ abstract class ChipAttributes {
 ///  * <https://material.io/design/components/chips.html>
 abstract class DeletableChipAttributes {
   // This class is intended to be used as an interface, and should not be
-  // extended directly.
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory DeletableChipAttributes._() => null;
 
   /// The icon displayed when [onDeleted] is set.
@@ -275,7 +277,8 @@ abstract class DeletableChipAttributes {
 ///  * <https://material.io/design/components/chips.html>
 abstract class CheckmarkableChipAttributes {
   // This class is intended to be used as an interface, and should not be
-  // extended directly.
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory CheckmarkableChipAttributes._() => null;
 
   /// Whether or not to show a check mark when [selected] is true.
@@ -309,7 +312,8 @@ abstract class CheckmarkableChipAttributes {
 ///  * <https://material.io/design/components/chips.html>
 abstract class SelectableChipAttributes {
   // This class is intended to be used as an interface, and should not be
-  // extended directly.
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory SelectableChipAttributes._() => null;
 
   /// Whether or not this chip is selected.
@@ -417,7 +421,8 @@ abstract class SelectableChipAttributes {
 ///  * <https://material.io/design/components/chips.html>
 abstract class DisabledChipAttributes {
   // This class is intended to be used as an interface, and should not be
-  // extended directly.
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory DisabledChipAttributes._() => null;
 
   /// Whether or not this chip is enabled for input.
@@ -464,7 +469,8 @@ abstract class DisabledChipAttributes {
 ///  * <https://material.io/design/components/chips.html>
 abstract class TappableChipAttributes {
   // This class is intended to be used as an interface, and should not be
-  // extended directly.
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory TappableChipAttributes._() => null;
 
   /// Called when the user taps the chip.

packages/flutter/lib/src/material/colors.dart

@@ -192,6 +192,9 @@ class MaterialAccentColor extends ColorSwatch<int> {
 /// The [Colors.transparent] color isn't shown here because it is entirely
 /// invisible!
 class Colors {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   Colors._();
 
   /// Completely invisible.

packages/flutter/lib/src/material/elevation_overlay.dart

@@ -14,7 +14,9 @@ import 'theme.dart';
 /// This is an internal implementation class and should not be exported by
 /// the material package.
 class ElevationOverlay {
-
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   ElevationOverlay._();
 
   /// Applies an elevation overlay color to a surface color to indicate

packages/flutter/lib/src/material/feedback.dart

@@ -80,6 +80,9 @@ import 'theme.dart';
 /// ```
 /// {@end-tool}
 class Feedback {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   Feedback._();
 
   /// Provides platform-specific feedback for a tap.

packages/flutter/lib/src/material/icons.dart

@@ -59,6 +59,9 @@ import 'package:flutter/widgets.dart';
 ///  * [IconButton]
 ///  * <https://design.google.com/icons/>
 class Icons {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   Icons._();
 
   // Generated code: do not hand-edit.

packages/flutter/lib/src/painting/matrix_utils.dart

@@ -11,6 +11,9 @@ import 'basic_types.dart';
 
 /// Utility functions for working with matrices.
 class MatrixUtils {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   MatrixUtils._();
 
   /// Returns the given [transform] matrix as an [Offset], if the matrix is

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

@@ -30,8 +30,9 @@ enum CacheExtentStyle {
 /// the framework recognize such render objects and interact with them without
 /// having specific knowledge of all the various types of viewports.
 abstract class RenderAbstractViewport extends RenderObject {
-  // This class is intended to be used as an interface with the implements
-  // keyword, and should not be extended directly.
+  // This class is intended to be used as an interface, and should not be
+  // extended directly; this constructor prevents instantiation and extension.
+  // ignore: unused_element
   factory RenderAbstractViewport._() => null;
 
   /// Returns the [RenderAbstractViewport] that most tightly encloses the given

packages/flutter/lib/src/semantics/semantics_service.dart

@@ -18,6 +18,9 @@ import 'semantics_event.dart' show AnnounceSemanticsEvent, TooltipSemanticsEvent
 /// When possible, prefer using mechanisms like [Semantics] to implicitly
 /// trigger announcements over using this event.
 class SemanticsService {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   SemanticsService._();
 
   /// Sends a semantic announcement.

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

@@ -23,6 +23,9 @@ class ClipboardData {
 
 /// Utility methods for interacting with the system's clipboard.
 class Clipboard {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   Clipboard._();
 
   // Constants for common [getData] [format] types.

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

@@ -11,6 +11,9 @@ import 'system_channels.dart';
 /// This API is intentionally terse since it calls default platform behavior. It
 /// is not suitable for precise control of the system's haptic feedback module.
 class HapticFeedback {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   HapticFeedback._();
 
   /// Provides vibration haptic feedback to the user for a short duration.

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

@@ -31,6 +31,9 @@ import 'platform_channel.dart';
   'This feature was deprecated after v1.6.5.'
 )
 class BinaryMessages {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   BinaryMessages._();
 
   /// The messenger which sends the platform messages, not null.

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

@@ -9,6 +9,9 @@ import 'platform_channel.dart';
 
 /// Platform channels used by the Flutter system.
 class SystemChannels {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   SystemChannels._();
 
   /// A JSON [MethodChannel] for navigation.

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

@@ -213,6 +213,9 @@ List<String> _stringify(List<dynamic> list) => <String>[
 /// Controls specific aspects of the operating system's graphical interface and
 /// how it interacts with the application.
 class SystemChrome {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   SystemChrome._();
 
   /// Specifies the set of orientations the application interface can

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

@@ -8,6 +8,9 @@ import 'system_channels.dart';
 
 /// Controls specific aspects of the system navigation stack.
 class SystemNavigator {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   SystemNavigator._();
 
   /// Removes the topmost Flutter instance, presenting what was before

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

@@ -15,6 +15,9 @@ enum SystemSoundType {
 /// Provides access to the library of short system specific sounds for common
 /// tasks.
 class SystemSound {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   SystemSound._();
 
   /// Play the specified system sound. If that sound is not present on the

packages/flutter/lib/src/widgets/route_notification_messages.dart

@@ -9,6 +9,9 @@ import 'navigator.dart';
 
 /// Messages for route change notifications.
 class RouteNotificationMessages {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   RouteNotificationMessages._();
 
   /// When the engine is Web notify the platform for a route change.

packages/flutter_test/lib/src/test_async_utils.dart

@@ -45,6 +45,9 @@ class _AsyncScope {
 /// });
 /// ```
 class TestAsyncUtils {
+  // This class is not meant to be instatiated or extended; this constructor
+  // prevents instantiation and extension.
+  // ignore: unused_element
   TestAsyncUtils._();
   static const String _className = 'TestAsyncUtils';