Add Slider.showValueIndicator property. (flutter#179661)

Closes flutter#179660

### Description

- Adds `Slider.showValueIndicator` property
- Adds test to verify that `Slider.showValueIndicator` takes priority
over `SliderTheme.showValueIndicator`

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

---------

Co-authored-by: Qun Cheng <36861262+QuncCccccc@users.noreply.github.com>

Author: ksokolovskyi

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

@@ -189,6 +189,7 @@ class Slider extends StatefulWidget {
     this.autofocus = false,
     this.allowedInteraction,
     this.padding,
+    this.showValueIndicator,
     @Deprecated(
       'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
       'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
@@ -216,7 +217,7 @@ class Slider extends StatefulWidget {
   ///
   /// If a [CupertinoSlider] is created, the following parameters are ignored:
   /// [secondaryTrackValue], [label], [inactiveColor], [secondaryActiveColor],
-  /// [semanticFormatterCallback].
+  /// [semanticFormatterCallback], [showValueIndicator].
   ///
   /// The target platform is based on the current [Theme]: [ThemeData.platform].
   const Slider.adaptive({
@@ -240,6 +241,7 @@ class Slider extends StatefulWidget {
     this.focusNode,
     this.autofocus = false,
     this.allowedInteraction,
+    this.showValueIndicator,
     @Deprecated(
       'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
       'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
@@ -568,6 +570,13 @@ class Slider extends StatefulWidget {
   /// overlay shape, whichever is larger.
   final EdgeInsetsGeometry? padding;
 
+  /// Determines the conditions under which the value indicator is shown.
+  ///
+  /// If [Slider.showValueIndicator] is null then the
+  /// ambient [SliderThemeData.showValueIndicator] is used. If that is also
+  /// null, defaults to [ShowValueIndicator.onlyForDiscrete].
+  final ShowValueIndicator? showValueIndicator;
+
   /// When true, the [Slider] will use the 2023 Material Design 3 appearance.
   /// Defaults to true.
   ///
@@ -914,7 +923,8 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
       thumbShape: sliderTheme.thumbShape ?? defaults.thumbShape,
       overlayShape: sliderTheme.overlayShape ?? defaults.overlayShape,
       valueIndicatorShape: valueIndicatorShape,
-      showValueIndicator: sliderTheme.showValueIndicator ?? defaultShowValueIndicator,
+      showValueIndicator:
+          widget.showValueIndicator ?? sliderTheme.showValueIndicator ?? defaultShowValueIndicator,
       valueIndicatorTextStyle: valueIndicatorTextStyle,
       padding: widget.padding ?? sliderTheme.padding,
       thumbSize: sliderTheme.thumbSize ?? defaults.thumbSize,

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

@@ -112,7 +112,8 @@ class SliderTheme extends InheritedTheme {
 }
 
 /// Describes the conditions under which the value indicator on a [Slider]
-/// will be shown. Used with [SliderThemeData.showValueIndicator].
+/// will be shown. Used in [Slider.showValueIndicator] and
+/// [SliderThemeData.showValueIndicator].
 ///
 /// See also:
 ///

packages/flutter/test/material/slider_test.dart

@@ -4210,6 +4210,62 @@ void main() {
     variant: TargetPlatformVariant.desktop(),
   );
 
+  testWidgets('showValueIndicator takes priority over theme', (WidgetTester tester) async {
+    Widget buildApp({
+      required ShowValueIndicator? themeShowValueIndicator,
+      required ShowValueIndicator? sliderShowValueIndicator,
+    }) {
+      return MaterialApp(
+        home: Material(
+          child: Center(
+            child: SliderTheme(
+              data: SliderThemeData(
+                valueIndicatorColor: Colors.red,
+                showValueIndicator: themeShowValueIndicator,
+              ),
+              child: Slider(
+                value: 0.5,
+                label: '0.5',
+                onChanged: (double newValue) {},
+                showValueIndicator: sliderShowValueIndicator,
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    void checkValueIndicator({required bool isVisible}) {
+      // _RenderValueIndicator is the last render object in the tree.
+      final RenderObject valueIndicatorBox = tester.allRenderObjects.last;
+      final PaintPattern matcher = paints
+        ..path(color: Colors.red)
+        ..paragraph();
+      expect(valueIndicatorBox, isVisible ? matcher : isNot(matcher));
+    }
+
+    await tester.pumpWidget(
+      buildApp(themeShowValueIndicator: ShowValueIndicator.never, sliderShowValueIndicator: null),
+    );
+    checkValueIndicator(isVisible: false);
+
+    await tester.pumpWidget(
+      buildApp(
+        themeShowValueIndicator: ShowValueIndicator.never,
+        sliderShowValueIndicator: ShowValueIndicator.alwaysVisible,
+      ),
+    );
+    checkValueIndicator(isVisible: true);
+
+    await tester.pumpWidget(
+      buildApp(
+        themeShowValueIndicator: ShowValueIndicator.alwaysVisible,
+        sliderShowValueIndicator: ShowValueIndicator.never,
+      ),
+    );
+    checkValueIndicator(isVisible: false);
+  });
+
   testWidgets('Event on Slider should perform no-op if already unmounted', (
     WidgetTester tester,
   ) async {