Use platform-specific back icon in Scaffold (#6182)

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

@@ -614,8 +614,19 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
       } else {
         _shouldShowBackArrow ??= Navigator.canPop(context);
         if (_shouldShowBackArrow) {
+          IconData backIcon;
+          switch (Theme.of(context).platform) {
+            case TargetPlatform.android:
+            case TargetPlatform.fuchsia:
+              backIcon = Icons.arrow_back;
+              break;
+            case TargetPlatform.iOS:
+              backIcon = Icons.arrow_back_ios;
+              break;
+          }
+          assert(backIcon != null);
           leading = new IconButton(
-            icon: new Icon(Icons.arrow_back),
+            icon: new Icon(backIcon),
             alignment: FractionalOffset.centerLeft,
             onPressed: () => Navigator.pop(context),
             tooltip: 'Back' // TODO(ianh): Figure out how to localize this string

packages/flutter/test/material/scaffold_test.dart

@@ -238,4 +238,39 @@ void main() {
 
     expect(appBarBottomRight, equals(sheetTopRight));
   });
+
+  group('back arrow', () {
+    Future<Null> expectBackIcon(WidgetTester tester, TargetPlatform platform, IconData expectedIcon) async {
+      GlobalKey rootKey = new GlobalKey();
+      final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
+        '/': (_) => new Container(key: rootKey, child: new Text('Home')),
+        '/scaffold': (_) => new Scaffold(
+            appBar: new AppBar(),
+            body: new Text('Scaffold'),
+        )
+      };
+      await tester.pumpWidget(
+        new MaterialApp(theme: new ThemeData(platform: platform), routes: routes)
+      );
+
+      Navigator.pushNamed(rootKey.currentContext, '/scaffold');
+      await tester.pump();
+      await tester.pump(const Duration(seconds: 1));
+
+      Icon icon = tester.widget(find.byType(Icon));
+      expect(icon.icon, expectedIcon);
+    }
+
+    testWidgets('Back arrow uses correct default on Android', (WidgetTester tester) async {
+      await expectBackIcon(tester, TargetPlatform.android, Icons.arrow_back);
+    });
+
+    testWidgets('Back arrow uses correct default on Fuchsia', (WidgetTester tester) async {
+      await expectBackIcon(tester, TargetPlatform.fuchsia, Icons.arrow_back);
+    });
+
+    testWidgets('Back arrow uses correct default on iOS', (WidgetTester tester) async {
+      await expectBackIcon(tester, TargetPlatform.iOS, Icons.arrow_back_ios);
+    });
+  });
 }