range_slider_test.dart diagnostics property tests (#40291)

* Updates to slider_test.dart

Fixed error that required onChanged.

* Updated slider.dart and slider_test.dart debugFillProperties

Updated slider.dart debugFillProperties to include all properties and slider_test debugFillProperties to test all properties

* Updated Slider onChanged property

* Updated debugFillProperties to ignore testing methods that are null in test

Per feedback from Hans/Anthony on Flutter Material

* Updated spacing in debugFillProperties

Per feedback from Hans

* Updated range_slider.dart and range_slider_test.dart to have DiagnosticProperties tests for all properties

* Updated range_slider.dart and range_slider_test.dart per feedback

Preventing null labels from creating an error.

Added made RangeLabels and RangeValues constants in instantiation of Range Slider.

Fixed spacing.

* Updates to start and end labels in diagnostic property tests

Updated per feedback from @clocksmith

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

@@ -337,6 +337,24 @@ class RangeSlider extends StatefulWidget {
 
   @override
   _RangeSliderState createState() => _RangeSliderState();
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(DoubleProperty('valueStart', values.start));
+    properties.add(DoubleProperty('valueEnd', values.end));
+    properties.add(ObjectFlagProperty<ValueChanged<RangeValues>>('onChanged', onChanged, ifNull: 'disabled'));
+    properties.add(ObjectFlagProperty<ValueChanged<RangeValues>>.has('onChangeStart', onChangeStart));
+    properties.add(ObjectFlagProperty<ValueChanged<RangeValues>>.has('onChangeEnd', onChangeEnd));
+    properties.add(DoubleProperty('min', min));
+    properties.add(DoubleProperty('max', max));
+    properties.add(IntProperty('divisions', divisions));
+    properties.add(StringProperty('labelStart', labels?.start));
+    properties.add(StringProperty('labelEnd', labels?.end));
+    properties.add(ColorProperty('activeColor', activeColor));
+    properties.add(ColorProperty('inactiveColor', inactiveColor));
+    properties.add(ObjectFlagProperty<ValueChanged<RangeValues>>.has('semanticFormatterCallback', semanticFormatterCallback));
+  }
 }
 
 class _RangeSliderState extends State<RangeSlider> with TickerProviderStateMixin {

packages/flutter/test/material/range_slider_test.dart

@@ -1533,5 +1533,40 @@ void main() {
 
     await gesture.up();
   });
+
+  testWidgets('Range Slider implements debugFillProperties', (WidgetTester tester) async {
+    final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
+
+    RangeSlider(
+      activeColor: Colors.blue,
+      divisions: 4,
+      inactiveColor: Colors.grey,
+      labels: const RangeLabels('lowerValue', 'upperValue'),
+      max: 100.0,
+      min: 0.0,
+      onChanged: null,
+      onChangeEnd: null,
+      onChangeStart: null,
+      semanticFormatterCallback: null,
+      values: const RangeValues(25.0, 75.0),
+    ).debugFillProperties(builder);
+
+    final List<String> description = builder.properties
+      .where((DiagnosticsNode node) => !node.isFiltered(DiagnosticLevel.info))
+      .map((DiagnosticsNode node) => node.toString()).toList();
+
+    expect(description, <String>[
+      'valueStart: 25.0',
+      'valueEnd: 75.0',
+      'disabled',
+      'min: 0.0',
+      'max: 100.0',
+      'divisions: 4',
+      'labelStart: "lowerValue"',
+      'labelEnd: "upperValue"',
+      'activeColor: MaterialColor(primary value: Color(0xff2196f3))',
+      'inactiveColor: MaterialColor(primary value: Color(0xff9e9e9e))'
+    ]);
+  });
 }