[Material] Fix slider track shape to rounded (flutter#34178)

The slider track shape for the onPrimaryColors constructor should be rounded, but it was accidentally changed to rectangular in flutter#31681. This change restores the original behavior. This only affects sliders that are themed with SliderThemeData.onPrimaryColors().

Author: clocksmith

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

@@ -417,7 +417,7 @@ class SliderThemeData extends Diagnosticable {
       overlayShape: const RoundSliderOverlayShape(),
       tickMarkShape: const RoundSliderTickMarkShape(),
       thumbShape: const RoundSliderThumbShape(),
-      trackShape: const RectangularSliderTrackShape(),
+      trackShape: const RoundedRectSliderTrackShape(),
       valueIndicatorShape: const PaddleSliderValueIndicatorShape(),
       rangeTickMarkShape: const RoundRangeSliderTickMarkShape(),
       rangeThumbShape: const RoundRangeSliderThumbShape(),

packages/flutter/test/material/slider_theme_test.dart

@@ -163,6 +163,30 @@ void main() {
     expect(sliderTheme.valueIndicatorTextStyle.color, equals(customColor4));
   });
 
+  testWidgets('SliderThemeData generates correct shapes for fromPrimaryColors', (WidgetTester tester) async {
+    const Color customColor1 = Color(0xcafefeed);
+    const Color customColor2 = Color(0xdeadbeef);
+    const Color customColor3 = Color(0xdecaface);
+    const Color customColor4 = Color(0xfeedcafe);
+
+    final SliderThemeData sliderTheme = SliderThemeData.fromPrimaryColors(
+      primaryColor: customColor1,
+      primaryColorDark: customColor2,
+      primaryColorLight: customColor3,
+      valueIndicatorTextStyle: ThemeData.fallback().accentTextTheme.body2.copyWith(color: customColor4),
+    );
+
+    expect(sliderTheme.overlayShape, const RoundSliderOverlayShape());
+    expect(sliderTheme.tickMarkShape, const RoundSliderTickMarkShape());
+    expect(sliderTheme.thumbShape, const RoundSliderThumbShape());
+    expect(sliderTheme.trackShape, const RoundedRectSliderTrackShape());
+    expect(sliderTheme.valueIndicatorShape, const PaddleSliderValueIndicatorShape());
+    expect(sliderTheme.rangeTickMarkShape, const RoundRangeSliderTickMarkShape());
+    expect(sliderTheme.rangeThumbShape, const RoundRangeSliderThumbShape());
+    expect(sliderTheme.rangeTrackShape, const RoundedRectRangeSliderTrackShape());
+    expect(sliderTheme.rangeValueIndicatorShape, const PaddleRangeSliderValueIndicatorShape());
+  });
+
   testWidgets('SliderThemeData lerps correctly', (WidgetTester tester) async {
     final SliderThemeData sliderThemeBlack = SliderThemeData.fromPrimaryColors(
       primaryColor: Colors.black,