Introduce Material 3 year2023 flag to SliderThemeData (#159721)

Related to [Introduce Material 3 `year2023` flag to the updated widget
themes](flutter/flutter#159484)


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Author: TahaTesser

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

@@ -192,7 +192,7 @@ class Slider extends StatefulWidget {
       'Use SliderTheme to customize the Slider appearance. '
       'This feature was deprecated after v3.27.0-0.1.pre.'
     )
-    this.year2023 = true,
+    this.year2023,
   }) : _sliderType = _SliderType.material,
        assert(min <= max),
        assert(value >= min && value <= max,
@@ -238,7 +238,7 @@ class Slider extends StatefulWidget {
       'Use SliderTheme to customize the Slider appearance. '
       'This feature was deprecated after v3.27.0-0.1.pre.'
     )
-    this.year2023 = true,
+    this.year2023,
   }) : _sliderType = _SliderType.adaptive,
        padding = null,
        assert(min <= max),
@@ -557,17 +557,18 @@ class Slider extends StatefulWidget {
   /// overlay shape, whichever is larger.
   final EdgeInsetsGeometry? padding;
 
-  /// When true, the [Slider] will use the 2023 Material 3 design appearance.
+  /// When true, the [Slider] will use the 2023 Material Design 3 appearance.
+  /// Defaults to true.
   ///
-  /// Defaults to true. If false, the [Slider] will use the latest Material 3
-  /// Design appearance, which was introduced in December 2023.
+  /// If this is set to false, the [Slider] will use the latest Material Design 3
+  /// appearance, which was introduced in December 2023.
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
     'Use SliderTheme to customize the Slider appearance. '
     'This feature was deprecated after v3.27.0-0.1.pre.'
   )
-  final bool year2023;
+  final bool? year2023;
 
   final _SliderType _sliderType ;
 
@@ -801,8 +802,9 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
   Widget _buildMaterialSlider(BuildContext context) {
     final ThemeData theme = Theme.of(context);
     SliderThemeData sliderTheme = SliderTheme.of(context);
+    final bool year2023 = widget.year2023 ?? sliderTheme.year2023 ?? true;
     final SliderThemeData defaults = switch (theme.useMaterial3) {
-      true => widget.year2023
+      true => year2023
         ? _SliderDefaultsM3Year2023(context)
         : _SliderDefaultsM3(context),
       false => _SliderDefaultsM2(context),

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

@@ -299,6 +299,11 @@ class SliderThemeData with Diagnosticable {
     this.padding,
     this.thumbSize,
     this.trackGap,
+    @Deprecated(
+      'Use SliderTheme to customize the Slider appearance. '
+      'This feature was deprecated after v3.27.0-0.2.pre.'
+    )
+    this.year2023,
   });
 
   /// Generates a SliderThemeData from three main colors.
@@ -623,6 +628,21 @@ class SliderThemeData with Diagnosticable {
   /// Defaults to 6.0 pixels of gap between the active and inactive tracks.
   final double? trackGap;
 
+ /// Overrides the default value of [Slider.year2023].
+  ///
+  /// When true, the [Slider] will use the 2023 Material Design 3 appearance.
+  /// Defaults to true.
+  ///
+  /// If this is set to false, the [Slider] will use the latest Material Design 3
+  /// appearance, which was introduced in December 2023.
+  ///
+  /// If [ThemeData.useMaterial3] is false, then this property is ignored.
+  @Deprecated(
+    'Use SliderTheme to customize the Slider appearance. '
+    'This feature was deprecated after v3.27.0-0.2.pre.'
+  )
+  final bool? year2023;
+
   /// Creates a copy of this object but with the given fields replaced with the
   /// new values.
   SliderThemeData copyWith({
@@ -661,6 +681,7 @@ class SliderThemeData with Diagnosticable {
     EdgeInsetsGeometry? padding,
     MaterialStateProperty<Size?>? thumbSize,
     double? trackGap,
+    bool? year2023,
   }) {
     return SliderThemeData(
       trackHeight: trackHeight ?? this.trackHeight,
@@ -698,6 +719,7 @@ class SliderThemeData with Diagnosticable {
       padding: padding ?? this.padding,
       thumbSize: thumbSize ?? this.thumbSize,
       trackGap: trackGap ?? this.trackGap,
+      year2023: year2023 ?? this.year2023,
     );
   }
 
@@ -744,6 +766,7 @@ class SliderThemeData with Diagnosticable {
       padding: EdgeInsetsGeometry.lerp(a.padding, b.padding, t),
       thumbSize: MaterialStateProperty.lerp<Size?>(a.thumbSize, b.thumbSize, t, Size.lerp),
       trackGap: lerpDouble(a.trackGap, b.trackGap, t),
+      year2023: t < 0.5 ? a.year2023 : b.year2023,
     );
   }
 
@@ -784,6 +807,7 @@ class SliderThemeData with Diagnosticable {
       padding,
       thumbSize,
       trackGap,
+      year2023,
     ),
   );
 
@@ -830,7 +854,8 @@ class SliderThemeData with Diagnosticable {
         && other.allowedInteraction == allowedInteraction
         && other.padding == padding
         && other.thumbSize == thumbSize
-        && other.trackGap == trackGap;
+        && other.trackGap == trackGap
+        && other.year2023 == year2023;
   }
 
   @override
@@ -872,6 +897,7 @@ class SliderThemeData with Diagnosticable {
     properties.add(DiagnosticsProperty<EdgeInsetsGeometry>('padding', padding, defaultValue: defaultData.padding));
     properties.add(DiagnosticsProperty<MaterialStateProperty<Size?>>('thumbSize', thumbSize, defaultValue: defaultData.thumbSize));
     properties.add(DoubleProperty('trackGap', trackGap, defaultValue: defaultData.trackGap));
+    properties.add(DiagnosticsProperty<bool>('year2023', year2023, defaultValue: defaultData.year2023));
   }
 }