Fix sort indicator for DataTables (#62795)

* Fix sort indicator for DataTables

As per material spec ascending order should be shown via an upward arrow
This commit changes the displayed arrows accordingly.

* Test sort indicator orientation in DataTable

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

@@ -610,7 +610,7 @@ class DataTable extends StatelessWidget {
       return onSort == null ? const <Widget>[] : <Widget>[
         _SortArrow(
           visible: sorted,
-          down: sorted ? ascending : null,
+          up: sorted ? ascending : null,
           duration: _sortArrowAnimationDuration,
         ),
         const SizedBox(width: _sortArrowPadding),
@@ -924,13 +924,13 @@ class _SortArrow extends StatefulWidget {
   const _SortArrow({
     Key key,
     this.visible,
-    this.down,
+    this.up,
     this.duration,
   }) : super(key: key);
 
   final bool visible;
 
-  final bool down;
+  final bool up;
 
   final Duration duration;
 
@@ -947,7 +947,7 @@ class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {
   Animation<double> _orientationAnimation;
   double _orientationOffset = 0.0;
 
-  bool _down;
+  bool _up;
 
   static final Animatable<double> _turnTween = Tween<double>(begin: 0.0, end: math.pi)
     .chain(CurveTween(curve: Curves.easeIn));
@@ -972,7 +972,7 @@ class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {
       ..addListener(_rebuild)
       ..addStatusListener(_resetOrientationAnimation);
     if (widget.visible)
-      _orientationOffset = widget.down ? 0.0 : math.pi;
+      _orientationOffset = widget.up ? 0.0 : math.pi;
   }
 
   void _rebuild() {
@@ -993,12 +993,12 @@ class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {
   void didUpdateWidget(_SortArrow oldWidget) {
     super.didUpdateWidget(oldWidget);
     bool skipArrow = false;
-    final bool newDown = widget.down ?? _down;
+    final bool newUp = widget.up ?? _up;
     if (oldWidget.visible != widget.visible) {
       if (widget.visible && (_opacityController.status == AnimationStatus.dismissed)) {
         _orientationController.stop();
         _orientationController.value = 0.0;
-        _orientationOffset = newDown ? 0.0 : math.pi;
+        _orientationOffset = newUp ? 0.0 : math.pi;
         skipArrow = true;
       }
       if (widget.visible) {
@@ -1007,14 +1007,14 @@ class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {
         _opacityController.reverse();
       }
     }
-    if ((_down != newDown) && !skipArrow) {
+    if ((_up != newUp) && !skipArrow) {
       if (_orientationController.status == AnimationStatus.dismissed) {
         _orientationController.forward();
       } else {
         _orientationController.reverse();
       }
     }
-    _down = newDown;
+    _up = newUp;
   }
 
   @override
@@ -1036,7 +1036,7 @@ class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {
                              ..setTranslationRaw(0.0, _arrowIconBaselineOffset, 0.0),
         alignment: Alignment.center,
         child: Icon(
-          Icons.arrow_downward,
+          Icons.arrow_upward,
           size: _arrowIconSize,
           color: (Theme.of(context).brightness == Brightness.light) ? Colors.black87 : Colors.white70,
         ),

packages/flutter/test/material/data_table_test.dart

@@ -4,9 +4,12 @@
 
 // @dart = 2.8
 
+import 'dart:math' as math;
+
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:vector_math/vector_math_64.dart' show Matrix3;
 
 import '../rendering/mock_canvas.dart';
 import 'data_table_test_utils.dart';
@@ -309,6 +312,54 @@ void main() {
     expect(tester.takeException(), isNull);
   });
 
+  testWidgets('DataTable sort indicator orientation', (WidgetTester tester) async {
+    Widget buildTable({ bool sortAscending = true }) {
+      return DataTable(
+        sortColumnIndex: 0,
+        sortAscending: sortAscending,
+        columns: <DataColumn>[
+          DataColumn(
+            label: const Text('Name'),
+            tooltip: 'Name',
+            onSort: (int columnIndex, bool ascending) {},
+          ),
+        ],
+        rows: kDesserts.map<DataRow>((Dessert dessert) {
+          return DataRow(
+            cells: <DataCell>[
+              DataCell(
+                Text(dessert.name),
+              ),
+            ],
+          );
+        }).toList(),
+      );
+    }
+
+    // Check for ascending list
+    await tester.pumpWidget(MaterialApp(
+      home: Material(child: buildTable(sortAscending: true)),
+    ));
+    // The `tester.widget` ensures that there is exactly one upward arrow.
+    Transform transformOfArrow = tester.widget<Transform>(find.widgetWithIcon(Transform, Icons.arrow_upward));
+    expect(
+      transformOfArrow.transform.getRotation(),
+      equals(Matrix3.identity())
+    );
+
+    // Check for descending list.
+    await tester.pumpWidget(MaterialApp(
+      home: Material(child: buildTable(sortAscending: false)),
+    ));
+    await tester.pumpAndSettle();
+    // The `tester.widget` ensures that there is exactly one upward arrow.
+    transformOfArrow = tester.widget<Transform>(find.widgetWithIcon(Transform, Icons.arrow_upward));
+    expect(
+      transformOfArrow.transform.getRotation(),
+      equals(Matrix3.rotationZ(math.pi))
+    );
+  });
+
   testWidgets('DataTable row onSelectChanged test', (WidgetTester tester) async {
     await tester.pumpWidget(
       MaterialApp(