Use adaptive more icon for popup_menu (#70773)

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

@@ -1102,20 +1102,6 @@ class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
     }
   }
 
-  Icon _getIcon(TargetPlatform platform) {
-    assert(platform != null);
-    switch (platform) {
-      case TargetPlatform.android:
-      case TargetPlatform.fuchsia:
-      case TargetPlatform.linux:
-      case TargetPlatform.windows:
-        return const Icon(Icons.more_vert);
-      case TargetPlatform.iOS:
-      case TargetPlatform.macOS:
-        return const Icon(Icons.more_horiz);
-    }
-  }
-
   bool get _canRequestFocus {
     final NavigationMode mode = MediaQuery.maybeOf(context)?.navigationMode ?? NavigationMode.traditional;
     switch (mode) {
@@ -1146,7 +1132,7 @@ class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
       );
 
     return IconButton(
-      icon: widget.icon ?? _getIcon(Theme.of(context).platform),
+      icon: widget.icon ?? Icon(Icons.adaptive.more),
       padding: widget.padding,
       tooltip: widget.tooltip ?? MaterialLocalizations.of(context).showMenuTooltip,
       onPressed: widget.enabled ? showButtonMenu : null,

packages/flutter/test/material/popup_menu_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:ui' show window, SemanticsFlag;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
@@ -368,8 +369,8 @@ void main() {
 
   testWidgets('PopupMenuButton is horizontal on iOS', (WidgetTester tester) async {
     Widget build(TargetPlatform platform) {
+      debugDefaultTargetPlatformOverride = platform;
       return MaterialApp(
-        theme: ThemeData(platform: platform),
         home: Scaffold(
           appBar: AppBar(
             actions: <Widget>[
@@ -405,6 +406,8 @@ void main() {
 
     expect(find.byIcon(Icons.more_vert), findsNothing);
     expect(find.byIcon(Icons.more_horiz), findsOneWidget);
+
+    debugDefaultTargetPlatformOverride = null;
   });
 
   group('PopupMenuButton with Icon', () {