Use horizontal dots on iOS menu button (#9722)

As required by
<https://material.io/guidelines/platforms/platform-adaptation.html#platform-adaptation-platform-recommendations>.

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

@@ -543,11 +543,23 @@ class _PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
     });
   }
 
+  Icon _getIcon(TargetPlatform platform) {
+    assert(platform != null);
+    switch (platform) {
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+        return const Icon(Icons.more_vert);
+      case TargetPlatform.iOS:
+        return const Icon(Icons.more_horiz);
+    }
+    return null;
+  }
+
   @override
   Widget build(BuildContext context) {
     if (widget.child == null) {
       return new IconButton(
-        icon: const Icon(Icons.more_vert),
+        icon: _getIcon(Theme.of(context).platform),
         padding: widget.padding,
         tooltip: widget.tooltip,
         onPressed: showButtonMenu,

packages/flutter/test/material/popup_menu_test.dart

@@ -13,7 +13,7 @@ void main() {
         routes: <String, WidgetBuilder> {
           '/next': (BuildContext context) {
             return const Text('Next');
-          }
+          },
         },
         home: new Material(
           child: new Center(
@@ -29,15 +29,15 @@ void main() {
                       const PopupMenuItem<int>(
                         value: 1,
                         child: const Text('One')
-                      )
+                      ),
                     ];
-                  }
+                  },
                 );
-              }
-            )
-          )
-        )
-      )
+              },
+            ),
+          ),
+        ),
+      ),
     );
 
     await tester.tap(find.byKey(targetKey));
@@ -55,4 +55,39 @@ void main() {
     expect(find.text('One'), findsNothing);
     expect(find.text('Next'), findsOneWidget);
   });
+
+  testWidgets('PopupMenuButton is horizontal on iOS', (WidgetTester tester) async {
+    Widget build(TargetPlatform platform) {
+      return new MaterialApp(
+        theme: new ThemeData(platform: platform),
+        home: new Scaffold(
+          appBar: new AppBar(
+            actions: <Widget>[
+              new PopupMenuButton<int>(
+                itemBuilder: (BuildContext context) {
+                  return <PopupMenuItem<int>>[
+                    const PopupMenuItem<int>(
+                      value: 1,
+                      child: const Text('One')
+                    ),
+                  ];
+                },
+              ),
+            ],
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(build(TargetPlatform.android));
+
+    expect(find.byIcon(Icons.more_vert), findsOneWidget);
+    expect(find.byIcon(Icons.more_horiz), findsNothing);
+
+    await tester.pumpWidget(build(TargetPlatform.iOS));
+    await tester.pumpAndSettle(); // Run theme change animation.
+
+    expect(find.byIcon(Icons.more_vert), findsNothing);
+    expect(find.byIcon(Icons.more_horiz), findsOneWidget);
+  });
 }

packages/flutter_test/lib/src/finders.dart

@@ -89,6 +89,16 @@ class CommonFinders {
   /// nodes that are [Offstage] or that are from inactive [Route]s.
   Finder byType(Type type, { bool skipOffstage: true }) => new _WidgetTypeFinder(type, skipOffstage: skipOffstage);
 
+  /// Finds widgets by searching for widgets with a particular icon data.
+  ///
+  /// Example:
+  ///
+  ///     expect(find.byIcon(Icons.inbox), findsOneWidget);
+  ///
+  /// If the `skipOffstage` argument is true (the default), then this skips
+  /// nodes that are [Offstage] or that are from inactive [Route]s.
+  Finder byIcon(IconData icon, { bool skipOffstage: true }) => new _WidgetIconFinder(icon, skipOffstage: skipOffstage);
+
   /// Finds widgets by searching for elements with a particular type.
   ///
   /// This does not do subclass tests, so for example
@@ -428,6 +438,21 @@ class _WidgetTypeFinder extends MatchFinder {
   }
 }
 
+class _WidgetIconFinder extends MatchFinder {
+  _WidgetIconFinder(this.icon, { bool skipOffstage: true }) : super(skipOffstage: skipOffstage);
+
+  final IconData icon;
+
+  @override
+  String get description => 'icon "$icon"';
+
+  @override
+  bool matches(Element candidate) {
+    final Widget widget = candidate.widget;
+    return widget is Icon && widget.icon == icon;
+  }
+}
+
 class _ElementTypeFinder extends MatchFinder {
   _ElementTypeFinder(this.elementType, { bool skipOffstage: true }) : super(skipOffstage: skipOffstage);